Where's "here"? I save a ton with the T-Mobile plan I'm on when I buy my phone outright. Paying roughly $1,200 over a two year period ($50/month including price of phone) instead of the $2,360 ($98.33/month including price of phone) over the same period if I was under contract.
....and yeah, as someone else said, a once carrier tied iPhone can definitely be unlocked (and for no charge).
The first part of you post confuses me. Even if you buy a phone from T-Mobile on the payment plan you still pay the same over 2 years or if you pay it up front -- Unless the last part you mean in contract with another carrier (like AT&T).