It will cost more than 20 quad extra surely?
Depends on how much you use it, and how much power it will draw, but using the following numbers:
200W increased useage vs current gen
2h per day of gaming/gpu load
Roughly 200 days of useage per year
Price of kWh = ~£0.30
Usage per day = 200W * 2h = 400W or 0.4kWh
kWh per year (increase over existing gen) = 200 x 0.4 = 80kWh
Total increased costs = 80 * 0.3 = £24