In reply to aln:
Last time I measured my PC (not in your league) it was about 120 W all told, including router. An Xbox, when we last had one that worked, was about 200 W.
Cost per hour is the power in W, divided by 1000, multiplied by your tariff (p per kWh).
So if we had been paying 16 p per kWh when that Xbox worked, it would have been costing us
16 x (200/1000) = 3.2 p per hour.
2 or 3 p per hour, most likely.