First of all the title of the anime and the 
characters themselves matter of cse.....the more 
popular they are, the more expensive. It's as 
simple as that. Which explains why a very simple 
shot of Nuriko from FY TV can cost so much more 
*sweatdrops* than a great shot from a less 
popular anime like for e.g. St Tail's.... In 
fact, I've seen wonderful cels (matchin 
BG/sketches/centred shot etcetc) from "lesser" 
anime going for dirt cheap prices and still no 
one wants...The effort put into drawing and 
painting it is probably the same or more yet 
there's such a big difference in prices!

Cels from memorable scenes and particular epi are 
also very valuable and sought after..e.g. 
Nuriko's death scene *sweatdrops again* or 
Escaflowne's Van with wings...So basically to 
understand the worth of the cels, you have to 
watch the show.

If u are comparin the same character cels, then 
the difference is more obvious, it's base mostly 
on aesthetical appeal. Upfront facial shots are 
more expensive than profiles/ Centred shots are 
better than off-centred ones/Opened eyes are 
better than closed eyes/Colored ones are better 
than monotone ones etcetc...Generally a cel that 
gives a clear view of wats going on in it is 
highly valued.

Pan cels are worth more generally coz they tend 
to show more ...like a full length portrait for 
e.g. But standard size cels can be juz as nice 
too....it all depends on the individual cels. 
Those featured in artbks/magazines/posters are 
called hanken cels and these are expensive becoz 
they are one of a kind, drawn solely for 
promotional purposes, unlike production cels 
which usually come in a set of sequences. The 
quality is also higher since they have to be 
displayed. 

Of cse how you value it is most important but 
there'll always be a market value for that cel 
and this could be different from what u think it 
is worth. So my suggestion is for you to look ard 
at the prices at ebay and online cel shops 
(there's a reason why some cels never get bought 
even if they are nice and why some get snatched 
up almost immediately)to haf a rough idea.

Welcome to the cel community....be prepared..this 
hobby requires a lot of $$ *sigh* and some luck 
too but its the hunt for cels tt makes it fun, u 
never know wats going to come your way :)