Each mirostock agency has a different pricing structure. Some are a flat rate, and for some, it depends on a variety of factors, from image file size down to when the buyer purchsed their credits.... Here's my real data breakdown, for the sites on which I'm active.
Shutterstock - $0.252 per image
iStock - $0.722 per image
Fotolia - $0.784 per image
Dreamstime - $0.955 per image
Lucky Oliver - $0.700 per image
123RF - $0.523 per image (but they're new)
ImageCatalog - $0.500 per image
SnapVillage - $0.00 (still in beta)
SS has the lowest $/sale, but offsets it with highest volum. They pay $0.25/image, plus $0.05 any time somebody gets a backup CD. They have a commission increase when you reach a certain level of sales.
IS seems to have no rhyme or reason in their pricing, anywhere from $0.20 up. But the average seems pretty consistent.
FT is $0.33 per credit, with files costing 1, 2 or 3 credits based on size.
DT has the highest payout. That's probably why some people do very well with them. But my overall sales are pretty mediocre, and I've had problems building my portfolio with them.
LO has a price structure similar to LO's, but only $0.30 per credit
123RF is still pretty new, not really enough sales to judge..