IPhone cracked back repair cost?

My brother cracked the glass back of my iPhone 4 and I was wondering how much it cost to fix. Also if I color the cracks and apply clear nail polish to the back to avoid shards, will the Apple store still fix it later on?
This is what I mean- image

If your contract is up, just do a new 2 year contract and get a new iphone 4 free.

You can also recycle your old phones or ipods on apple's site and they will give you store credit for it, so at least you will get something back for it. Even if damaged something is better then nothing.
http://www.apple.com/...recycling/

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