My back shattered, I went to the apple store with no genius appointment and pleaded with one of the sales guy for help. He politely told me it would be 199 dollars for an out of warranty replacement. I told him how disappointed I was that this happened, because my fall was a not a particularly drastic one, and it was wearing the stupid bumper at the time. He took it in back for about 5 minutes, then came back and said "I'm going to go ahead and do an out of warranty replacement for you but waive the 199 fee". I jumped for joy and signed all the papers and went home, and bought a real case.
So, first I'd ask apple nicely. You may or may not get lucky and get a free replacement. Failing that, you have two options, buy a new backplate and replace it yourself for 149 plus shipping. Or pay apple 199 for an out of warranty replacement in store.