Last year, I ordered 2 iPhone 6 models (64GBs). I opted for AT&T's NEXT 12 (20 monthly payments). I've made 12 payments thus far @ $37.45/piece.
Over the course of 12 months, I've paid $449.40 ($37.45 x 12 months) per phone. After 12 payments, I owed $299.60 per phone.
I just sold both phones on nextworth.com for $390 a piece (promo code GETNEWIPHONE). So, I owed $299.60 but sold them for $390. So instead of giving my phones back to AT&T after 12 months to upgrade, I just paid them off and sold them to Nextworth and pocketed the $180 ($90 per phone).
Another way to look at it. I paid $449.40 over the course of 1 year to use each phone. I got $390 back for each phone. So I basically spent $59.40 to use each phone for 1 year!
That last paragraph doesn't work without the $299.60 you still owed.