I could be wrong but I think pads will last longer with your stoptechs compared to your OEM brakes since the you have a larger rotor not to mention ST's aero rotor is supposed to help cooling.
I think PF01s run about $350 for the ST40s which is inline with what PF01s cost for other pad shapes.
2013 F30 328i Melbourne Red
2011 E90 M3 Monte Carlo Blue
2004 E46 M3 Imola Red