To make the math a little clearer:
500 PetaHash/second = 500,000,000 GigaHash/second
500,000,000 GigaHash/second * 0.1 Joules/GigaHash * 0.03 USD/(KiloWatt*hour) * 24 hour/day * 365 day/year * 1/1000 KiloWatt/Watt = $13M/year (remember Watt = Joules/second, so the terms cancel)
15% of $13M is about $2M