How much does it cost to unlock an iphone 4?

Recently was given an iPhone 4 (not 4s) and it won't accept my 3 mobile micro-sim how much would it cost to get it working on my 3 contract?

Added (1). To the guy who said "search the internet" that's what i did… And i landed here >.>

I have no idea search on net.

Take it to a Mobile Repair shop, they can unlock it.
usually about £20

The cost of unlock depends on the network to which the mobile is locked to. If you can get it from the network provider, sometimes they arrange it for free. If not, opt for a reliable third party unlock. There are many available in the internet… But I have used the unlock from http://www.onlinegsmunlock.com/apple-iphone-4/rs7wp9/ for my phone and unlocked permanently through their remote unlock service. You can visit their site and if satisfied with the rate get your iPhone 4 unlocked through them…