How much does the "$50" iPhone 3GS from AT&T really cost?

*Except as follows, two-year contract required. No term required for tablets, SIM cards or accessories. Tablets require minimum $14.99/mo data plan. AT&T service required for SIM card. Other restrictions apply." I saw the AT&T ad for the $50 iPhone 3GS and thought it was too good to be true, but when I looked it up on their site i saw that fine print. I wondered what the actual cost was altogether and didn't have my "Business Jargon Decoder Ring" nearby to figure it out. So if i already have an existing plan with AT&T can i just walk in with $50 and walk out with an iPhone 3GS?