A cel is essentially worth what someone will pay 
for it... yes that sounds vague but let me 
explain!  In general, a close-up shot with eyes 
open and mouth closed will be more valuable.  A 
Key cel is more valuable than a regular cel.  
Pencil sketches increase the value slightly and 
backgrounds increase it significantly.  HOWEVER!  
You can't say that just because something is a 
long shot of the back of someone's head that it's 
worthless, or that all closeups with the eyes open 
and mouth shut will sell for hundreds of dollars. 
 It also depends on the character, series, and 
scene.  For instance, a closeup of Kenshin will 
sell for around $300 (like the one I just won on 
eBay, yay!) while a similar shot of Fancy Lala 
might go for $10 if the seller is lucky.  New 
shows like Tenshi ni Narumon are a great way to 
get started--Tenshi is such a cute show but its 
still new enough so that a lot of people haven't 
seen it, so even great cels of adorable characters 
are still reasonable.  Good cels from older shows 
(Macross, Captain Harlock, etc) will be 
significantly higher because the shows are well 
known, and many of the cels have been destroyed or 
damaged over the years.  Within a show, different 
characters will be more valuable too, based on 
popularity and rarity.  Using Kenshin as an 
example again, Yahiko isn't worth too much, 
Kenshin is worth a lot, and Yumi... oh god I wish 
I had one!  ^_^

Well... I hope that all makes sense... the best 
advice I can give as far as buying is: If YOU thin 
kits worth it, go for it, if not, don't!  And 
don't worry too much about what other people 
think.  Some of my cheapest cels have got me the 
most compliments.  And for some reason everyone 
love my $7 Wal-Mart sweater... go figure... ^_^  
Hope that helped!

Tifa