150 Mhs gets you roughly 3 USD per day, electricity cost = 0,1 USD/kwh , 0,75 ( 750 watt ) x 24 x 0,1 = 1,8 USD electricity  cost , 3-1,8= 1,2 USD profit. Is it sensible to build a mining rig with this conditions? Maybe with second hand parts, definetaly not with brand new equipment. Is it sensible to continiue mining with your existing rig? Yes