While there are thousands of animation cels used in a movie, the problem is those who have them not taking care of them. Line fading, being stuck to paper, etc can all easily happen if they aren't stored or displayed properly, which is why they are sometimes hard to find.
The more popular the character, the more expensive the cel is going to be. If it has a background, it's also worth a bunch. If you have a cel of a random background character, it probably won't be more than 50.
A popular character, or a group of characters will probably be 100-200. With background 200-400 is a good range.
The cels currently on eBay
(search here) are overpriced in my opinion (especially the one with the background as it's not the original background)
I bought a group cel (actually 4 separate cels that all are from the same scene) in great condition of Hazel, Blackberry, Bigwig, and Fiver last year for 150.