There are several ways that value can be determined, eye of the beholder being almost first and foremost.
But there are, in terms of cels, several different types.
First of all, there are production cels. These are the cels which are one-of-a-kind, actually used in the making of the original productions, whether it's for a movie, a tv show or for some other special purpose, for example, to add animation to a commercial or an awards program. These are hand-drawn, hand-inked and hand-painted. The last full-length Disney movie to use exclusively production cels was The Little Mermaid.
Since that time, all Disney movies have used mostly computer animation, which means that for all computer generated animation, no original one-of-a-kind production cel exists. Production Cels from movies, depending on the content, usually claim the highest prices. (On the other hand, some scenes just don't lend themselves to much value as art by themselves, or of minor characters, will not command a high price. A friend of mine has a cel of "miscellaneous fish" from Bedknobs and Broomsticks - really cute, but people aren't beating down the doors to get it.) Original producation cels from older movies can reach very high, like Snow White, one of which got over $300,000 at auction a few years back.
TV production cels are farily widely available, still are one-of-a-kind, and the prices are much more affordable. You can get some of these at Art of Disney for about the same price as a sericel.
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Then there are cels which were never used directly for a movie or show, which Disney has produced strictly for the collectors. the sericels and other limited edition cels. Disney even releases one-of-a-kind cels from the computer generated movies. Usually once or twice a year they have an auction of these type of cels from their most recent movies.
The value of these depend on the edition sizes, the subject, the background (a 'created' cel may still have a 'production' background, which is a valuable work of art in and of itself.)
Generally, the sericels have the largest editions, and as a result are the more affordable. These cels are not hand done, they are printed on the cel through a serigraph process. Other limited edition cels may have the lines printed but the actual paint hand-done, or may have the entire cel hand done. Again, depending on the edition size and the subject, the each of these types of cels can sell out and then the secondary market would determine the value. Sericels usually retail between $250-$750, the 'limited edtion' cels usually run higher. (There is a stated edition size for the sericels so they are actually 'limited editions, too)
Each of these cels, whether a sericel or a production cel, is usually a true copy of a particular scene in the show.
Lithographs, on the other hand, which are generally on paper, tend to be artisti interpretations in and of themsleves, and are not usually a particiular scene in a show.
I hope this helps. If I got any part of this wrong, please correct me.
