I think "They are not to much money compared to 
non-anime" is waaay too general (after all, we all 
know both Western cels and anime cels have a huge
price range) ^_- 

The problem is that you asked about Mononoke in 
particular - cels from this are *extremely* rare. 
I've personally only seen maybe half a dozen, many
in Japanese galleries, and the ones I've seen on 
sale are $1500-3000 for lesser B-grades. Perhaps 
they haven't been officially released. Ghibli 
tends to take the top rung for A-level cels, 
certainly (besides other obvious picks like Lodoss 
OAV, Evangelion, etc.). Expect mid-upper quadruple 
digits for better quality Mononoke relative to 
current pricing, if you ever find one. Also 
considering tendency to go to auctions. ^_^;