The cost to develop a drug is actually a matter of considerable debate. I'll agree that $100M is a lower-end figure for it. Most of the cost depends on how many failed candidates you factor into the calculations. The cost to actually develop a successful drug is probably $50M or so. The problem is that for every one of those you have many failures, but most of those tend to be abandoned before the full $50M gets spent.
My point is just that MRSA isn't commonplace enough to be a priority for private investment. People will pay $25k for a cancer treatment, but not for an antibiotic. Antibiotics aren't taken chronically either.
Don't get me wrong - I'm all for spending more on antibiotics. The money should come from taxes, and the resulting antibiotics should be illegal to prescribe except when there is evidence that other antibiotics would be effective (or something to this effect, I'm more than happy to let the experts define the appropriate diagnostic criteria but I certainly don't want to see the NIH spend $1B on a MRSA cure only to see it put in "hand soap").