A pricing guide is pretty much impossible for a 
lot of reasons.  One reason is that no two cels 
are exactly the same, so you would have to price 
every cel in the series - obviously that's not 
going to happen with thousands of cels per 
series.  Prices also vary widely based on when 
the cel is sold, and where.  A cel sold on a 
private collector's site will often not bring as 
much as one on a popular dealer's site.  Auctions 
can drive a price higher if bidders have a 
bidding war and two or more bidders decide to go 
just a *little* higher... often several times.
The scene the cel comes from is also very 
important.  Two cels which are essencial 
identical can range a lot in price based on the 
outfit the character is wearing.  The condition 
of the cel also affects the price- paint missing, 
line fading, folds in the acetate.  The economy 
has also been affecting prices, lots of cels 
aren't selling for as much because people just 
don't have as much spending money.  The 
popularity of a series can change- ie, the price 
of the cels can increase up to about ten fold 
once the show is released in the U.S. or shown on 
cartoon network.  Lastly there is an uncertain 
supply at best- if the studio doesn't release any 
cels for years, the cels become scarce and prices 
increase.