Hi,
Here's a little help: check out
www.kimonocels.com/Animation/production.html
for basics(it's all in the last point
"Grading cel and pricing").The only other "basic"
which comes in to my mind would be the
condition of the cel matierial(don't know if it 
was mentioned in the link above):It shouldn't
have been dammaged anyway.E.g.if you cut or
crack or whatever pieces off the cel
can loose an extremly high part of it's worth. 
Well, even then you have to think about it some
time because the whole pricing thing
is ,well, quite subjective(you can
try to find out how other people priced
cels more or less similar to yours:).
hope this helps a little.