In my experience, if you buy an unlocked phone, you hand the carrier a bonus payment of the cost of the phone. They don't give you any cheaper service.
And, when I went to replace a Nokia POS that died 3 days after warranty, I discovered that Verizon even subsidizes their prepaid phones: you can get an LG VX5600 for $40 with new service, but to replace the Nokia with the existing account, they wanted $200. (Allegedly, they "don't sell" the Nokia in question as a prepaid, and thus couldn't offer a direct replacement; which raises the question of why phones they "don't sell" come with their branding and work on their network.)