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?

Im looking at purchasing a HTC inspire at target for 49.99$ for a new account. Will that be the price i pay if i am a new Att customer? also why is it at the att store 150 online 100 and target only 50???? Can i then buy the phone at target and then go to the att store to get the plan and everything? plus does att add extra costs monthly to your bill even if you pick certain features. for example i am looking for 450 mins 200mb data unlim texting comes out to around 75$ a month, will that stay consistant?