I won't come back on everything people said about 
basic economy : costs to go to Japan, run a shop, 
salary, unsold cels,...  as for every business
those are to be taken into account when setting 
prices or you won't run your business for long.

But I also want to point out something that wasn't 
mentioned : You can't run a cel business by just 
reselling the few cels you bought from Mandarake.
Yes, Mandarake sometime has very interesting 
prices but you usualy can only buy a very few cels 
from them. Also, they are now selling most of the 
interesting one on auction.

No, most cels that dealers are selling have to 
come from elsewhere. Mostly from japanese 
collectors or people working in the animation 
business. And those people are not stupid : they 
are not going to sell at low prices if they can 
make more money by auctioning the cels themselves.
That mean that if a dealer want to buy those cels, 
he will have to pay the price.

Finaly, there is no absolute way of setting prices 
for cels. It happened several time that during the 
same trip I bought two cels from the same 
sequence, nearly identic, without any differance 
of quality. But one costed the double of the 
other. Simply because they were not bought from 
the same person. Why did I bought the most 
expensive one ? Because I considered that even at 
that price, it was still interesting...

Cheers,
Yann Stettler