For the writer of contemporary novels, there is no substitute for the research trip. Only then does the author him or herself see, feel, smell, hear the locale with original senses, not through the editing of others' reportage. The author will meet people who provide the pieces of authentic characters, and with luck will be able to stay long enough to pick up the cadence of speech and the pacing of life. Writing without it is strictly second choice.
For the historical writer, the research trip's usefulness is in direct proportion to the distance in time to the chosen period.
Don't think that you can't properly do a historical novel without a trip to the places you describe. Especially if yours is set more than a couple of hundred years ago, it is better to spend $500 on hard-to-get books than $5000 travelling to get much less relevant information.
An actual trip to the physical location can enhance book work, but taking a week's vacation cannot take the place of real research.
In researching the near past, a research trip alone can give you a feeling for climate, but not much else. For example, more than a half-century has passed since WW2. Take a trip to LA for your novel, and you will see more things that should NOT be in your book than should. Shipyards have changed equipment, if not location. The Vincent Thomas Bridge crosses the harbour, where in 1944 ferries chugged back and forth, closer to the ocean than the bridge stands. Smog constantly obscures what were then common vistas. Freeways web the landscape where once there were only surface streets laced with streetcar tracks. Buildings then of note are gone, then-illegal skyscrapers towering in their stead.
You can hunt out bits and pieces of WW2 LA, but only if you have researched to a fare-thee-well first. A really good picture-book or three of the period would tell you more. Your money would be better spent at the book-finder than the travel agent.
Research trips are most useful when you are targeting the 19th and perhaps 18th centuries, and you build your itinerary around Living History sites. These are the places like Old Tucson, San Diego's Old Town, Mystic Seaport, or Colonial Williamsburg, where an older way of life has been reconstructed with the buildings, and "re-enactors" in costume work at traditional crafts or present vignettes.
However, these are only in the 50% to 75% range of accuracy. Take one of the grandest of these projects, Colonial Williamsburg. The "hostesses" (docents) that explain the buildings and guide you through them wear hideously half-baked costumes: modern bodices with under-bust curves, the lines of their "hen-basket" dress supports showing through because they don't wear petticoats between wire and skirt. You get to hear the sound of horse-drawn carriages in the street, but miss the dust and smell of accumulated horse-droppings, not to mention outhouses in high summer. You don't get to see it unlit at night, dodging footpads. But you do get to see the arrangement of furniture in rooms, the dark-airy feel of government chambers, smell the wood-shavings accumulating in a workshop, and innumerable other details you might need: 75%.
The best way to exploit one of these sites is to do it once with the usual tourist crowd. Then come back to places you consider relevant, sit down and soak up the scene, then take copious notes on your responses and observations. Without the notes, you will surely misplace the flight of a line of crows, the scent of local weeds, the peculiar sound of wind in the sails of a mill, or other telling, concrete details. Finally, arrange to discuss items of interest with the re-enactors. If possible, get hands-on with them. Looking at a plow and actually trying to guide one behind a mule are two different levels of experience!
For older periods yet, the ratio of anachronism to re-experience climbs sharply. You must do as much research in order to guard yourself against the bad points of a visit as you would have had to have done without the trip. The main points on which actual site visitation may aid you are:
1) VISTAS. What can you see from the headland or tower, provided you can get up on them? Do the immemorial paths run on ridgetops or in the gullies? Remember that hills or islands may have been removed or altered naturally or artificially, that some lakes are very new, rivers had different courses, coastal land can be extended or destroyed in a century, and, of course, buildings shift. Good picture books or videos will often substitute nicely, though this is probably the hardest aspect of the area to reconstruct at remote.
2) AMBIANCE. How do thatched, half-timbered dwellings look in the sunset? What are the feel of ancient hills, the smell of the marshes? Surviving buildings that haven't been "modernized" (whether with 19th century improvements or wood floors to replace dirt) will be worth your time. Landscape often isn't. The area is either wetter or dryer, hotter or colder than it was then, deforestation has radically changed the hills, or reforestation is with different plants in different patterns. If the area used to be ancient oak forest, go find a modern, old oak forest, maybe on a different continent or much closer to home. It will often tell you more than travelling to your geographical target, only to find the forest replaced with farms, vineyards, or condominiums. However, if research shows that physical change has not been great, going there can help you get a feel for the locale, especially if you live in Wisconsin and want to write about Tahiti in the early days of European contact. You will still be able to pick up original but authentic impressions. But you can do without if you hit the photographic sources and read enough by people who have been there, or correspond with those who have.
Also, at restored or reconstructed sites, you must be wary and well-researched to know whether what is there was there in your target period. For example, at Olympia in Greece, the site reconstruction is the Hellenistic and Roman Imperial Olympia, with Stadium III. If you are writing about a runner in the Golden Age of Periclean Athens, the Echo Colonnade is sitting where the Stadium II finish line was. The dramatic covered entry for the athletes did not exist, and the Stadium had only three sides. At Knossos, you get the reconstruction of the final stages of the New Palace period, which largely obscures the Middle Palace and Old Palace palaces. Colonial Williamsburg is shown in its heyday, not the early, crude colony nor the decayed site of the 19th century.
3) MUSEUMS. If you make arrangements in advance, you may be able to sit for a week and study the collections. That sort of visit can beautifully cap your months of book work. Chugging through with the other tourists, museums can actually be misleading. Docents are talking to the average member of the public, who wants general cultural enrichment, not a writer tuned in to one era. At a certain museum in York, England, they will show you a piece of jewelry, and call it a "Viking torc." They do not tell you how completely different this is than a Celtic torc, and so you bedizen your Britannic princess with an item 800 years ahead of fashion, the equivalent of giving William the Conqueror a frock coat. Elsewhere, you will get shown "Medieval armor" that does not apply to your century in the Middle Ages but having just heard about a rood that does, you get confused easily and understandably. Otherwise, the docent may not mention - or know - that what she is pointing to is parade armor, not battle harness. Worse confusions attach to Egyptian artifacts, as Ancient Egypt is often treated as a monolithic culture, rather than variations on a theme for centuries. Some Gods were not worshipped at all periods, for example.
Books and laserdiscs of museum collections are sometimes better than the original, since unless you are allowed to handle the goods, visual data is all you would acquire through a visit. As well, you can stare as long as you like, for as many days as you like, at the photographs, rather than being hustled out by the docent after ninety seconds. What you do lack is the immediate impression of how large items are compared to you. Seeing a horse mannequin in barding from well off to one side where the photo was taken is not quite as dramatic as looking up and up at it from floor level. Standing in the temple of Karnak will give you a feeling of the scale no picture can.
4) LIBRARIES & BOOKSTORES. If you want to do heavy research on an area, you will find more regional books in the region than outside it. You can snag all sorts of pamphlets and booklets on old houses that no-one has heard of outside of town. You will be able to read the local paper for your target year. However, you can locate a lot of books online now, contact the town librarian by e-mail (or land mail) for recommendations or photocopies, and get a lot more done that you could have in the mere 48 hours you would have physically been able to stay. In fact, use your 48 to set up contact with that librarian or town historian for later in-depth work.
If you were going to take a vacation this expensive anyway, by all means make it a tax-deductible business expense (for you -- your companions' expenses don't count unless they, too, are writing books about the area)(Wouldn't that be a super custom charter tour to set up?). Just be sure to take the trip after you've done a lot of research, not right at the start. You won't know what to concentrate on, what to ignore as anachronisms, and will lose the wonderful feeling of recognition and discovery when you see what you've been studying, when the place in your head suddenly anchors itself over the present scene.
Just don't use the lack of a research trip as an excuse to deprive us of your novel any longer!