My PC uses 125w idle, with 99% GPU uses ~240w, Different is 115 watt/h.
For 24H running 2.7 kw. With 300 MHash/s i earn 0.0072 BTC / 24H, this is 0.54 Euro. My power cost is 0.23 EUR/kw.
0.621 Euro power cost and 0.54 Euro BTC earing. That is -0.081 Euro pro day.
This not worth it to mining with unused GPU time.
Your calculation is using current exchange rate. This makes sense when you are planing to sell/spend your freshly mined btcs immediately. Otherwise, exchange rate may change either upwards or downwards before you use your coins, you don't know in advance. Anyway, if you're going to use your coins and not sell them immediately, your calculation can't be that simple. :-)
It makes more seance to buy them at less cost them mined them... Right?