Most expensive production cel sale...$3,000 for a cel directed 
by the famous Miyazaki.  There are anime production cels that 
are more expensive, I just can't think of any right now.  
Judging the price of a cel goes by a combination of things...
like judging any piece of artwork.  I'm sure that someone 
else's post will be able to help you more.  :)  The appeal of 
certain cels to me is most often the scene where the cel 
came from, and how much I like the character.  For instance, 
the cel I have posted below is from the movie Adolescence 
Mokushiroku (a movie based on the series Shoujo Kakumei 
Utena) and of the character Anthy.  I believe cel is from 
just before the dance scene, a wonderous piece of Japanese 
animation which is beautiful and romantic, and definitely 
my favorite part in the movie.  When I look at this cel, I 
remember that scene...also, it is signed by one of the creators
 of the series, Saito Chiho, whom I greatly respect and 
admire.  Additionally, not many movie cels were released (initially
sold only at the movie's premier in Tokyo) which makes me 
proud to own it.  I recommend that you look at the links provided 
by cels.org and take a look cels for sale at the dealers 
listed http://www-scf.usc.edu/~ckawakam/cels/ (just 
don't look at Mangaya!  Way overpriced!)  One final note; 
cels for sale in North America go through a large markup, which is understandable because the dealers need to have enough 
money to keep a number of cels in stock, yet still make a profit.