Studios release cels however they feel like it. 
Of course, those involved with the production 
at a high level can probably take their pick; for 
instance, no one will tell Hayao Miyazaki, no, 
you can't have that one. I doubt they charge 
him. Lower down, it's possible that some 
employees may be able to purchase cels at a 
"discount" but I don't know for sure. Some are 
likely given away in a sort of promotional way, 
but the majority would be sold to "distributors" 
of sorts that then sell them, probably on 
auction in lots, to individual dealers. 

The studio gets to decide how many of their 
cels are released to be sold. Up until recently, 
when cel collecting became popular, the 
majority were destroyed. Back before the 
whole idea of "collectibles" came around, for 
instance in the early Disney productions, I 
know that people working in the ink&paint 
department could take cels they had painted 
home when the studio was done with them. 
They didn't have any value, in the sense of a 
market. And these were not "C" grade cels 
either; those were probably all destroyed. 

As for the studio making money, remember 
that the cost of an animated production is 
massive. Also, when a studio sells a cel, it 
won't be to the final owner. Each person the 
cel passes through does a mark-up, that's the 
nature of retail. So if the studio sells the cel at 
its market value, especially for successful 
productions, by the time it would reach anyone 
in the general populace it would be basically 
unaffordable. Unlike "genuine" art, there is a 
limit - high though it may be - to what a cel 
collector can pay. There are no cel museums 
willing to pay ridiculous amounts. 

One last thing - Mononoke cels are expensive 
because A) it was highly popular and B) there 
must have been very few cels released from it. 
This was probably something to do with many 
of the cels being distributed within the studio. I 
remember Miyazaki saying it was a particular 
favorite of his. Also part of releasing few cels 
may have something to do with a 
phenomenon which happened in the comics 
industry, a sort of collecting market crash. If 
you make them too readily available, the 
prices fall rapidly. In the minds of collectors, 
"rare" = "valuable". So in some ways, better to 
not release any "C" grade cels. 

Okay and now I'll stop rambling. ^_^ I'm not 
pretending to be an authority on all this . . . just 
my theories. So correct me when I'm wrong . . .

Ayaka