I've seen a lot more Last Unicorn cels than I have for any 
other Rankin Bass cartoon, or similar cartoons of that 
genre. The majority of Last Unicorn cels I've seen have been 
at animation art galleries that mark up cel prices much 
more than anime cel dealers, and there's a lot of demand to 
justify the prices. It's probably better for collectors for there 
to be higher prices anyways, because that can help more 
cels to surface private collections or the production crew. 

If you're looking for Last Unicorn drawings, The Deep 
Archives has some awesome stuff. Their cels are priced 
entirely too high, but they have a ton of interesting Rankin 
Bass stuff  http://www.thedeeparchives.com/anim.htm