For example, if your electricity costs $0.10/kWh and your GPU uses 100W then your power cost is $0.24 (or about 0.0024 BTC) per day. Currently, if your GPU's hash rate is 600 MH/s, then it mines about 0.0072 BTC per day, and your profit is 0.0048 BTC (or about $0.48) per day.