Well 

I have seen quite a few production Mononoke cels 
in Japan, the price range are about #150000 to 
#500000

they are just C to B quality cels.

Every studio release cels differently, I am very 
fimiliar with studio sunrise and studio pierrot.

For example studio pierrot, they release cels at 
their festival, and at character shows, several 
years ago, they used to release high quality cels 
at really low price, cels like Fushigi Yuugi were 
released at very low price.

However a lot has change in last two years, now 
days you can attend studio pierrot`s anime fest, 
and will only find over priced power stone, GTO 
cels and those #50000 each reproduction cels, 
they seems to have changed the way they release 
cels.

as for studio sunrise, they still release good 
quality cels at reasonable price.

If you think Japanese animation cels are cheap 
compare to American animation cels, just wait 
around 2 to 3 years, and watch the price climb, 
if you compare the price now to two years ago, it 
is kind of scary to imagine what price they will 
be in two years.