I would be glad to assist.  Cost is really 
subjective to the buyer and depends on factors 
such as image placement, scene, character, 
action, etc.  I uploaded 4 pages I printed out 
years ago from kimonocels.com (which is defunct) 
that I personally have always used to assess an 
anime cel.

http://s1204.photobucket.com/albums/bb413/jstride
r7/Anime%20Grading%20and%20Pricing/?
action=view¤t=CelPricing1.jpg

As for prices in my personal experience it's all 
up to how much you want to let your cels go 
for.  You might want more than you paid 
(investment), less (clearance) or the same 
(break even).  My suggestion is to check 
www.cels.org and go to keep track to see what 
other online dealers are asking for them.  Sadly 
most don't update often due to lower demand so 
www.Anime-Gambit.com (here on the forum) and 
www.animeed.com are the best to compare for 
pricing (in my humble opinion) as they regularly 
weekly update.  All sites are excellent places 
to find cels from different series.  Go to sites 
such as Ebay (check completed listings) and 
Rinkya (cels being sold in Japan) to see what 
others are asking for similar title and quality.

The anime cel scene has really changed in the 
last 10 years with the advent of computer 
renderings making the cel art all but obsolete 
so the value is and yet always has been truly in 
the eye of the beholder.
     John