nah you wouldn't be wasting the $650. you can get a 30 day refund, if the gamble of cutting a sim down to nano size and changing settings does not work. you just get another sim module, like from t-mobile or whoever.
tmobile has this deal where they give u a small amount of minutes but a lot of data... for like $30 a month.
i've heard in some t-mobile areas is metropcs compatible, so you can theoritcally metro pcs an iphone, but it would only work in very limited t-mobole gsm areas
there are a lot of at&t mvnos to choose from, gsm wise.. all the verizon mvnos require you to have a cdma device, like pageplus. they have a $10 bring your own device fee
i do not know about different LTE bands though
a lot of mvno companies selling pre paid service for iphones require you to actually buy the iphone from them, like boost mobile i think