how much to replace glass on back of iPhone 4

Depends on the country and if the Apple Store does this.

Call Apple and ask.

You can get it on amazon for as little as 13 bucks and it is only 2 screws to get it off. http://www.google.com/m/products/catalog?q=iphone+4+replacement+back+glass&oe=UT F-8&hl=en&client=safari&um=1&ie=UTF-8&tbm=shop&cid=14039850082016906812&sa=X&ei= UA-4T4L6CZKm8gSE6JiWCg&ved=0CHkQ8wIwAQ <----- link for 13 dollar plate

do NOT follow Swank's poor advice. This will void any remaining warranty with Apple. In the US, the fee is $29 to replace the back glass. Very reasonable, and won't void the warranty.

Would it be 60 to get the front and back then, the screen is fine it's just the other stuff is cracked

Sorry guess i meant 4s does that change the 29 dollars when you said thats how much for the 4