Pity you are at 0, because this is exactly it. Whenever I wish to change phone I look at the same phone on contract 'subsidised' and paying outright. Every time so far it has worked out cheaper to buy the phone, then equivalent service. Also every time my monthly cost has dropped after some time, so it works out even cheaper than the original calculation makes out.
Further - if i wish to change phone, I sell the old one, and get to choose a new one (I never come out ahead, but it is nice to reduce the burden).