Switch mode PSUs generally only draw as much power from the wall as you suck from them, so sucking more through the PC PSU isn't gonna save a cent. Only thing that would make a difference is the relative efficiencies of the PSUs.
I remember The RoadWarrior..To understand who he was, you have to go back to another time..the world was powered by the black fuel & the desert sprouted great cities..Gone now, swept away..two mighty warrior tribes went to war & touched off a blaze which engulfed them all. Without fuel, they were nothing..thundering machines sputtered & stopped..Only those mobile enough to scavenge, brutal enough to pillage would survive. The gangs took over the highways, ready to wage war for a tank of juice