Mononoke was wildly successfuly in Japan and in 
Asia (from what I understand). From 
www.nausicaa.net (a very useful site for all you 
Ghibli fans), Mononoke grossed $154 million at 
the box office in Japan alone. I also believe 
that the film broke a lot of records there until 
Titanic was released (editorial comment: yuck). 
So you can imagine that the Japanese fans would 
be more willing to pay a lot for these cels. 
Other countries are obviously much smaller 
markets.

Also from Nausicaa net:

Mononoke used 144,043 cels
Kiki's Delivery Service: 67,317 cels 
Nausicaa: 56,078 cels
Laputa: 69,292 cels
(Totoro and Porco Rosso cel counts currently not 
available).

Keep in mind though that many of these cels would 
be "C" level cels.

Wendy