IPhone 4S screen replacement?

I'm thinking of buying my friends iPhone 4S. There's absolutely nothing wrong with the phone, and it's fully functional. The only thing is, there's a few scratches on the phone's screen. It's not shattered, and the scratches aren't super noticeable, but I'm a bit of a perfectionist and they're an annoyance to me. Is there any store that would replace my screen for cheaper than the Apple store itself? I'd really like to stay under $100 if possible. Also, how difficult is it to replace the screen yourself? I've had no experience with that.

Here is a link if you would like to do it your self they have replacement kits for sale for 50$

http://www.iphone4parts.com/shop/

as for how difficult it is just look it up on youtube and you can decide for yourself.