The plastic covering is pretty cheap, however there are a few problems with buying the replacement.
To replace the screen you have to take the phone a part. I think for most providers this voids the warranty. Taking the phone a part is not that big of a deal though, it's pretty simple. Just make sure you do some research on how to do it, there's videos all over the place. Just remember this WILL most likely void your warranty, but as long as you do it right there is probably no way for them to know just by looking at it.
Also I'm not really sure how the screen is attached to the housing. I think it's stuck on by a very sticky glue or glue pad. You may have to replace the whole housing (or at least just the front piece along with the screen protector) in order to replace just the screen. I could be wrong I've not taken mine a part before, so someone correct me if I am wrong. But even then it's fairly cheap for the housing maybe like $10-20.
Do a search on you tube or google replacing blackberry screen and you will get directions. The screens are under $10.00 and you do not have to take your phone apart to replace them.
