You might want to look at this:
DPreview post on getting a cracked ring repair. Keep in mind this is the internet. You bought your lens used, correct? If so, well, then, we're not under warranty, but you'll still get an idea of how much a repair like this costs.
I think repairs like these depend on who you get working on your gear. El Segundo rebuilt my D2H for me after a nasty accident which nearly involved both me and the camera tumbling on asphalt (luckily, it was just the camera) for free, so you never know what you'll get.