The specific edition of the movie in question place a large role in its overall value. The initial release of the animated features on VHS--called "Black Diamonds" because of the black diamond which sits behind the Disney logo on the box--generates a great deal of attention in collector's circles. They're recognizable by their oversized clamshell covers, much larger than regular VHS covers. Disney DVD releases use similar labels--gold editions, platinum editions, special editions and anniversary editions--to denote exactly when a given DVD was released and thus how much it may be worth at a given time.
Disney uses the term "in the vault" or "moratorium" to describe films which they have taken out of circulation. Once that happens, the film disappears from video store shelves and becomes increasingly difficult to find (thus sending the price up). It has nothing to do with overall sales--Disney classics like "Snow White" and "The Lion King" are placed in the vault as well as more obscure titles. It's merely a marketing strategy intended to increase demand whenever the company re-releases a given title. In order to properly price an old Disney movie, you need to know which ones are out of print. The Ultimate Disney website maintains an up-to-date list, and you can get a general idea of available titles by checking Amazon and looking for titles listed as out-of-print.
Exact prices for old Disney movies fluctuate far more than other collectibles, since Disney artificially manipulates their availability. EBay remains a reliable site to gauge the current value of an old Disney movie; check existing auctions to see what people are paying for a given movie at the moment. The Disneyana fan club can also offer practical advice on gauging a given movie's price at the moment.
Some Disney movies receive a release overseas or are currently available in overseas format. This is another reason why proper identification of a given edition is so important, since it differentiates between foreign and domestic Disney releases. Overseas Disney tapes were likely printed in the PAL format, while DVDs use different region encoding that limits their playability on American equipment. Shipping costs may further increase the price.
Beware of frauds or movies in poor condition. The clamshell VHS cases could take quite a beating, and any movies purchased should still be viewable in your DVD player or VCR. In particular, watch out for anyone claiming to have an "authentic" version of the movie "Song of the South." Disney has refused to release the movie on VHS or DVD because of its perceived racial content: any "authentic" version is likely a bootleg.