Where to find the cheapest Iphone?

At the moment I have a blackberry which I got at my last upgrade and it finally died and no longer works. My upgrade is in May. But I need to find the cheapest way possible to get an Iphone. It doesn't matter what generation I just need to find a really cheap iphone.

The 4s iPhone i think is like free now

I would get one from ebay.

What I think is the smartest (and cheapest) thing to do is go on amazon or in gamestop and buy a used iphone (the 3G is like 100 in good condition while the 4 is 200 in good condition) and buy a net 10 SIM card and use the iphone with net 10 it's 50 a month for unlimited everything! I use them and could not be MORE satisfied. It's also a lot cheaper then getting a phone on contact which cost up to 100$ a month! Anyway hope I helped