Here's a quick overview on what I know:

"A" type portrait cels are often the most 
prized "standard" sized cels. However, bank cels 
(cels from sequences that are re-hashed; the 
Sailor Moon tranformation sequences are an 
example of this), tome cels (held still on screen 
for longer than the average cel), harmony cels 
(the cel is only lines with color on the 
background), and other specialty cels are often 
worth more.

Pan cels are often more highly valued; however, I 
find no basis of comparison between standard cels 
and pan cels, as the facotrs between what makes 
one better than the other are very subjective. A 
pan cel of a foot is worth less than a primo 
portrait shot on a standard cel, for example.

As for sketches, that is VERY dependant on what 
sketches you are referring to. Most of the time, 
they are considered as worth less, but there are 
instances where hand-drawn sketches have 
surpassed their cel counterparts.

To go any depper than that, I would have to go 
into character preference and show popularity, as 
well as condition of the cel, which are all 
things that affect a cel's value at any given 
time.

Hope this helps!

-Misha