Mononoke is arguably Mizayaki's best movie and is 
certainly one of his most popular.

Cels would be popular not only amongst regular 
cel collectors but also other more infrequent 
collectors of great movie memobrilia. Think about 
the box office take in Japan vs other anime 
movies.

Similar to a Disney cel from say Snow White where 
one would pay $10,000+ or so.

Moral: It is frustrating that the cels are 
expensive AND hard to find - after all if you 
bought one would you sell it? But those are the 
laws of limited supply vs big demand and very 
well heeled collectors. 

Either you're in or your out. Its no use hoping. 
Every collector has to decide their own pain 
threshold. In the case of Mononoke. Its obviously 
has to be high.