You first have to make the differance between
hentai cels and cels from hentai anime.

Many hentai anime have good design and are
of high quality. So there is many good cels
that aren't at all hentai in themselve. (Like
most of those posted in this thread).
Such cels are usualy cheaper than cels of the
same quality from other anime. Mostly because
hentai anime are less know. (many people buy
cels only from anime they watched).

Naturaly, there is exception. For example,
Word Worth is a recent hentai anime with very
good design and high quality. Cels from it
are quite expensive...

Slightly hentai cels (tastful nude) are often
more expensive than non-hentai one whatever
they are from an hentai anime or not. (Look
what happen if you auction a naked Ryoko from
Tenchi Muyo...).

The realy hentai (pornographic) cels aren't usualy 
worth much : they are nearly always of far lower 
quality. Don't forget that a A grade cel is nearly 
always a close-up (of the face of the character, 
you silly !), eyes open, with a nice expression.
A little hard for a truly hentai cel to fit
those criteria...

Some people, especialy outside of Japan, are
ready to pay high prices for pornographic cels.
But that's because they believe such cels are
hard to get and so valuable. That's a mistake :
in Japan, you can get whole bunch of them from
some shops (AWS has a part nearly only for
hentai cels). It's just that most dealers don't
import those cels. Whatever because they don't
want to sell pornographic cels or because they
don't think they are worth enough to bother.

Cheers,
Yann Stettler