Going back to the OP's situation then, you still haven't proven anything.  The OP hasn't even stated his electricity costs.  Not to mention, you're still running on the (possibly false) assumption that he is running four cards per rig, or has other means/methods of lowering the rig electricity draw per card to 25 watts.  Like I already pointed out, not everyone run rigs full of four cards each.  And for those of us that do not, our electric costs will be higher because of it.  So, without purchasing a new motherboard and more mining hardware, he very well could have no choice but to mine at $7 of electricity per BTC.