It's all depening on supply and demand.  If the
dealer sees that cel can sell for $1000, even
though he bought it for $20 many years ago, of
course, he'll sell it at $1000.  For example,
I know many dealers bought the Lodoss War or
Miyazaki series of cels many years ago.  They
kept them in their own collections for a long
time, and finally when the prices of the cels
reach new highs, they are willing to let go for
profits.  Dealer is here to make money regardless
the percentage of mark-up.  If that series of
cels have good market prices, they will mark up
high to make money.  Most of the dealers that I
have seen outside of Japan, whether selling in
the conventions or the internet have marked up
high on most of their cels.  Of course, the not
so popular series of cels are always priced cheap.