Depends on the character, condition, OVA or 
MOVIE, sketches (quite rare), original 
backgrounds and how nice the character looks in 
a particular cel. Both OVA and MOVIE cels go for 
a lot when it's a favorite character like 
Belldandy or a rare one like Peorth or Holly 
Bell. The pic below is just one example of a 
pretty rare and expensive cel from the AMG! OVA 
and the other pic I will reply is another 
example of an expensive AMG! Movie cel. Hope 
this information helps.

Good Luck,
Phillip S.