A couple more considerations I left out:

1. It's winter here, so in a sense, I'm just using this power to heat my room
2. This is also putting wear on my hardware. That's probably actually the most significant expense and the most difficult to calculate.

Just got my first payout of 9 ETN earlier today. As interesting as the economic calculations are, there's something special about getting paid for running a program on your computer, and I'd say that's priceless!