Why do I have to pay full price for T- Mobile iPhone 5?

Ok so I went to the tmobile store to upgrade my old basic phone for the new iPhone 5 16gb and the rep told me that I had to pay the full cost ($579, i believe). On the ad it shows $99 for the 16gb! The plan I have right now is the Shared Classic Unlimited-Talk + Text, so I don't know if the plan is the problem or what? I also have no data added. Maybe that's why. Anyways if someone could help me I would really appreciate it.

Dial customer care and ask if you are upgrade eligible if so then ask how much it is they'll tell you on the phone also make sure it's not a Tmobile authorized dealer

I guess the price 99$ is for starting a new plan, so if you're only 1/2 way into your 2 year plan and it's just an upgrade and they can decide what phones/prices the upgrade will offer. T-Mobile sucks that way. My sisters boyfriend had it and had so many issues like them giving the same number out twice! So someone had his same exact phone number so all that persons calls came to his phone. So weird.

I had Verizon in the States with an iPhone 4 and it cost me 200$ even when it was my phone upgrade time. So not very cheap but much better than the full price! But i don't know if that was offered all the time or only a limited time or what. Why not ask how much it is for the iPhone 4?