What's the cheapest way to get an unlocked iPhone?

I know I'm still going to be paying a lot but what's the cheapest way to buy one? Amazon, eBay.ect?

Also what's the lowest generation you'd recommend? I was thinking 4s is the lowest even though its not that old but will all the apps still work on any less than that?

Well to save even more money you could get an AT&T iphone and get this chip from eBay for like $5 and it will unlock the phone. If I were you i would get 4s, or wait a couple months for Apple's new iphone, that way the iPhone 5 will be cheaper and you can buy iphone 5 instead of 4s