I don't know the details regarding the NEXT plan, but from what you're saying, you're paying $510 for a phone you don't keep, correct?
That's terrible, compared to what I've been doing since 2008. Buy a new iphone every year for 299 (now it's 399 due to memory and screen size), and sell my wife's 2 year old iphone for about 250-350 (depending on screen and memory size) and hand down my 1 year old iphone to her.
So that costs roughly 50-150 per year to upgrade, out of pocket, once you contact ATT for that BS "activation fee" to be waived.