I can't say for sure, but I'm guessing most of the large MSC are experienced enough to pad the fees so that they don't lose $. There are some companies that pay a pittance to begin with and routinely "bonus" their shops to get them done. In all reality...they are only paying a fair wage when they "bonus" them. They are just hoping there are people who like to shop and don't care about the $ and take the low fees. So if you see a company offer $10 and regularly have to increase and pay $20-$25 by the end of the month...you can be pretty sure they are still charging the client in excess of what they are paying the shopper.
On rare occasions I have seen the fee raised a substantial amount and that makes me think the MSC is penalized by the client for NOT delivering the information on all locations. I would think if I am a large chain (retail or restaurant) and am being sold "an information service" by a MSC, I would expect that the salesperson promising to provide the information would deliver on all locations and not just the easy urban locations that don't take much effort to fill due to the high volume of shoppers.