Getting a deal on a phone by renewing a phone on a family plan

My phone is the only one in my families plan that has an expired contract, so I've been assuming I can get a deal on a new phone, it seems I'm more or less wrong.

Through Amazon I can get the blackberry for free after upgrading my account, but I'd have to leave my family plan (I'd lose too much every month for it to be worth it in the end), and if I get it through Cingular, I can stay in the family plan, but their deals are no where near as good as Amazons (instead of free, it's $300, only $200 off).

I don't know the ins and outs of the system, or anything even close, but maybe I can get some words of wisdom on my situation.

As of now, I have a Motorola T720 which should have never seen the light of day.

Thanks guys.