Who sets the price for Über rides?

I've used the now-famous internetty car service Über a couple of times. The prices for the two rides I took were about 20-25% more expensive than taxis. Which made me wonder about how the prices are set for Über rides. Are they set in Silicon Valley by some all-knowing limousine wizard? Does it allow for competition, that is, can an enterprising Über-enabled driver choose to offer a lower price?

I worry because if Über actually becomes dominant they could through monopoly drive up the prices of black cars (which we used to call limousines, but that sounds too fancy I guess.) Could they in turn drive up the prices of regular taxi cabs?