I disagree with your maths. If the saving was 50/mo you'd break even in 6 months. The GTX 970 consumes 145 watt, the GTX 980 consumes 165 watt according to NVidia. In a total of 15 days of non stop full use, 360 hours, this translates to 52.2 kwh for the GTX 970 (USD 13.42, I pay usd 0.26/kwh) versus 59.4 kwh for the GTX 980 (USD 15.27). So the saving would be USD 1.85 per month, however if I'm not mistaken the GTX 980 is the better of the two so there is no 'saving' as such.