How is it cheaper to unlock?

This is one of those gray era questions that I am in doubt about so I will just ask. I played with an iphone and think I might have iphone fever. I will be asking questions that might seem stupid but this is the stuff I am wondering.

OK, I hear that people hate AT&T's $60 a month prices and have been unlocking their phones for T-mobile and other comanys.

I agree that T-mobile has better customer service and support and is an easyer company to deal with than AT&T. But isn't it way more expensive to do that? Because with AT&T, you get voice and data for the $60, ok and now you have T-mobile that has them as two seprate packages for about $40 a piece. So don't you save $20 a month for keeping the phone unlocked and with AT&T? Or are the unlockers just selecting one or the other, voice or data?

I'd guess a lot of people using unlocked iPhones are mainly using it for its voice & iPod features and limiting the data use to wifi networks.

I plan on getting one at some point (let's see what T-Mobile Germany decides to charge) and if I was to stick to my current plan, I'd get a 10€ voice plan plus a 10€ data plan for 250MB a month. The rest of the time I would use the wifi connection at university.

They don't do it becuase the monthly plans are cheaper. They do it becuase they dont want to pay cancelation fees or loose their t-mobil features. Mostly though the people doing this are not in the US and they do this becuase it is the only way they can use a IPhone.