Worst case: CPU only hashing with expensive electricity, 24/7 mining. (0.5 MHash/sec & $0.3kW/h).
Monthly profit: negative ~$42. So a loss of $500 annually.
Good case: High end GPU with cheap electricity (50 MHash/sec & $0.2kW/h).
Monthly profit: Positive ~$26. So a profit of $312 annually.
So if your high end PC cost $2500 it would be break even after 96 months. Assuming no hardware failures & Ethereum price doesn't change, etc.....
In a cool climate where you were going to run a heater to keep your house warm anyway, this might make sense. As you could also save on the heating costs. In a hot climate where you needed to run the A/C harder to cool the room, then it probably wouldn't make sense.
Best case: If you already had excess solar power that you couldn't sell to the grid at a fair price, it would make more sense. Maybe profit ~$35 / Month, depending on how sunny it was. So $420 annually. Payback on your PC in 71 months.