As I posted in another thread, GPU mining is still very profitable with paid-off hardware.

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With an existing 5850 GPU and summer air conditioning, it is costing me ~$0.80/day to mine (total rig wattage). (~$5.60/week)

At the current difficulty, I earn ~0.09333BTC/week which is currently ~$12/week.

That makes for a profit margin of ~100+% and is significantly cheaper than purchasing BTC directly with that ~$5.60/week.

At the current difficulty, it would be profitable for me to mine even with the price all the way down at $61/BTC.