There is 30.70 # of K2O in a DM ton of cornstalks and 4.47#P205 or 51.1# potash and 8.6# of map per ton of DM stalks.  A dry 5x6 stalk bale will be about 1200# and at 15% moisture will contain right at 1020# DM which means it takes 26.1# potash and 4.4# of MAP to replace the nutrients removed by the bale.  Using your numbers of 315#/acre and 52#/acre means 11.8-12 bales per acre; not sure what you are smoking, but I have never seen 170bu corn make 12 5x6 bales per acre.

I think you need to sharpen your pencil a little lot.  At 4 bales/acre for 170bu corn you would need 104.4# potash($21.54) and 17.6# MAP ($6.77) using your prices of $412.70/T and $769.23/T respectively.  That $28.31/ac is a far cry from the $85 you had figured; perhaps a new calculator is needed too.