What is a good price to pay for an iPhone 4 now (2nd hand)?

My friend is selling an iPhone 4 and he said to give an offer, but I dont know how much to offer.

I will need to pay £20 to get it unlocked so is £150 a reasonable offer? I honestly have no idea

Yes at that price… Its great. Recently I checked the apple web site and they provide $100-150 for a second user 4GS phone… That's a fairly good buy for no trouble.http://www.cellitused.com/

Look for average price on MrJump.co.uk:
http://www.mrjump.co.uk/classifieds/apple-iphone-4