I'm an Apple Tech, these are the prices they've told us but they're also available via the Apple Support webpage.
Edit - Sorry, thought you were referring to prices. Yes, our guides show that it's a whole-unit exchange. Same as on iPods and iPads. By the time you've taken them apart you've either damaged them or they won't likely go back together as seamlessly as before.
Thanks for your reply. I hope Apple reconsiders this over time. I expect there will be a lot of scratched sport crystals needing replacement and without an affordable solution sales will be impacted, imo.
I'll keep my fingers crossed.