$150 a month? That's a lot even by crazy standards. iPhone doesn't even have LTE.
Only problem is that iPhone, like most phones, I believe, doesn't support the t-mobile 1700 band. Which means no HSPA+ data; you'd be limited to EDGE. They are in the process of changing to the 1900 band, but distribution is limited. Straight Talk is another prepaid option that allows you to be carried by AT&T for $45; see that thread. Also, some of the other guys in that thread were pushing Solavei, which will supposedly be similar. I have no real idea about those two, however, because I too use the $30 t-mobile plan with my penta-band Nexus.