Hi, I want to get an iPhone eventually (probably will wait for the 5th gen), but anyhow, what is the cheapest plan I can possibly get? I went to the AT&T store and they said the least expensive is $15, with 200mb of internet. That seems too much for my taste (I know, I'm frugal). But then I saw Ebay and Craigslist and noticed many were buying their iphones off of there. If I were to buy it off of ebay/craigslist, can I just insert my SIM card, under a family data plan, in the iphone and get it to work as a phone? Thanks!