need help getting a motokrzr

ok, so here is my situation, i have a nextel i670 on a plan with my mom,she is paying for our nextel bill and my sisters sprint bill, in september it will be a year and i will get 75$ off a new phone(correct me if im wrong).
with this savings im switching back 2 sprint with my sister(nextel phones r butt ugly)
anyways i want 2 know how much it would cost
plzzzz help me out