Art isn't affected just by demand, though. When it is, it ceases to exist, as happened with the huge decline in sculpture and art technique from the Classic Roman Period through the Dark Ages; if people can't or aren't willing to buy artistic productions at a level that compensates the expense and effort that goes into it, then it ceases to exist and the knowledge of how to create it is lost, little by little generation by generation.
In an art market context 1992 doesn't really matter so much. This piece has a lofty background because of the artist. A similar piece made by a hobbyist who had no relation to the production of the series would be worth $1000-$2000 just because of its handcrafted-ness. A print or mechanical reproduction would be worth a couple hundred dollars.
During the Dark Ages, art was affordable only by churches no smaller than an abbey and the high nobility. That grew to include the gentry, guilds, and merchant princes during the Renaissance, finally attaining mass market status in the 19th century with a bourgeois middle class attempting to assert social parity with the old aristocracy. In general, since then, the value of a handcrafted piece of art (even one made by a more or less unknown artist, as most of them are) has been comparable in value to a designer couch.
Speaking generally, it has to be that way. The value of art is affected by the materials, craftsmanship (effort), and time (production hours) that go into it, including the expense of the education it took to develop the skills to create such a thing. $500 wouldn't even cover the cost of materials and is basically a gift price; you pay him for the materials and he gives you his time and effort for free.
This would be a very proud piece in a Codexer's man cave to shadow him and his friends during a night of D&D, every bit as valuable to a certain demographic as a designer couch is to a woman trying to impress her social circle and show her cultural distinctiveness.
