If your GB stated $290 it should be $290 and nothing more regardless of power consumption. Those were the terms the customer agreed to.
People paid for the SP30 only. The hosting was a bonus offered to anyone that wanted. It wasn't enforced or anything and I am sure that it's based on the power consumption. The good thing is that I don't think that it will vary a lot because we can't push the PSUs a lot more than 2700W.