It depends on what version of the i5 you have, and then it depends on what version of the i7 you want. It would probably cost you around 100$ bucks. Tell us what version you have and what version you want.
They're all correct! Now if you really need more performance you can buy a 6 core i7 for $600-$1000 which I'm not even sure that's compatible. Simple, if you want to upgrade it will be around $300 with around %10 performance increase.