I've tracked Disney movie cels for about a year 
and have some Disney cels, although I collect 
mostly anime. I'll answer the question based on 
the assumption that you're looking at movie cels 
since that's what I'm most familiar with.  

In general, I find anime cels are available 
closer to "street prices" than Disney cels. Many 
Disney movie cels are sold through dealer 
inventory or consignment. The difference between 
Disney retail to street is generally much higher 
than for anime. 

Although the answer depends on a lot of factors, 
I think it's the same considerations whether it's 
anime or animation:

1. Is the cel competitively priced (e.g. street 
vs dealer)? 

2. Is the show going to be popular among affluent 
collectors in the future (e.g. 10 years from 
now)? 

For example: If Studio Ghibli is still making 
animation 10 years from now, I think cels from 
Ghibli classics will hold up well compared to 
cels from Disney. But if Ghibli goes away, the 
cels may not hold value as well. On the other 
hand, it's not as if street prices for Disney 
cels haven't gone down either. 

Also, I suggest looking at historical auction 
close prices for the Disney cels over the last 5 
to 10 years. There is a site that tracks these 
for a small fee, but the cel restoration site 
also has prices for their auctions. 

If you need more info, feel free to email me. And 
if the cel is from The Little Mermaid movie and 
you don't get it, keep me in mind! :)

Good luck!