Since you asked, there are several reasons. First, a large part of OA's pricing is dependent upon nostalgia and that very definitely affects price. Fine art stands on its own: it can be appreciated, and will appreciate in price, without knowing the level of detail which separates, say, the first appearance of Wolverine from the artist's other work or a particular story line. Second, it is not dependent on carrying a portion of a story, like OA, which is a particular issue when getting to panel pages, and will definitely affect non-fans. Third, fine art is less dependent on the characters involved, although subject matter does matter (I expect Renoir's ballarinas would sell for more than, say, a picture of a dead parrot). Fourth, Toulouse-Lautrec aside, most of the market growth these days in fine art is not particularly representatonal, but more abstract. You can't really do that with OA. And a real biggie, there is the absence of color (which is making me suspect that those color guides may one day sell for more).
So no, in my view, Jim Lee's X-Men won't hang onto high prices in 30 years because the X-Men won't be that big a deal. Kind of like buying cowboy comic art today but on a much grander scale.