No 3G after screen replacement iPhone 4

Hello everyone!

Basically, I have a white iPhone 4. It's out of warranty, and the screen broke, so i got it replaced, however ever since my signal keeps on dropping (i seemed to have fixed it by resetting network settings) but when i put it in my pocket and take it out, it's always searching for network. However that's not my problem, ever since i got the screen replaced my 3G doesn't work, it doesn't detect 3G at all and it's all enabled. I tested my sim in my other phone and 3G works fine, i did let the shop know who replaced my screen so they will be looking into it, however i wanted to find out how much an out of warranty iphone would cost if i took this to apple?

Thanks!