Can I buy an AT&T phone (HTC Inspire) retailed at $49.99 and then take it to the AT&T store to activate it?

I am looking to buy the htc inspire from target for $50 instead of the att price of 150$ and the online price of 100$. If i am a new att customer can i buy the phone from target for that 50$?? Where do i get it activated and set the plan up and what not at target or do i need to go to the actual att store? Does att slip extra costs as you start paying monthly? I am looking for 450 mins with 200mb data plan and unlim texting. this comes to around 75$.