Actually, one F22 is better than ten F15s. I'll have to dig up some recent articles on the Red Flag and other excercises that F22s have taken part in. The kill ratio was something like 69-1 and that the loss was only because a Raptor pilot got ballsy and crossed the merge with three F16s. He killed all three but a heater from the last F16 got him too. In all the other engagements, the other planes couldn't even find the Raptors. Pilots transitioning from the F15 to the F22 couldn't believe how one-sided and easy the engagements became.
Another big factor is the cost of training and retaining the pilots, not mention the associated costs of support personnel, aircraft maintenance and other costs. The more planes and pilots you have, the greater the cost.