Assuming you're running at about 1.9 Mhps/Watt, that puts your average power-consumption at 1,750W (1.75kW) which is 1260kW per 30 days or $252 USD per month. 3.2 Ghps at current 1.69m difficulty @ $13.7 USD/btc is pretty close to $800/month. That's how I'm guessing how you're close to $550/month.
An assumption about the 1.9Mhps/watt can only be made if running old hardware that's very difficult to obtain (5xxx series cards).
My set up: AMD 145 CPU, mATX mobo, 2Gb RAM, 30Gb SSD, 6950 (cannot unlock shaders any more). 330Mhash/s, slightly overclocked, RAM downclocked to half, fans at 25%, 80Plus PSU. Using an accurate power meter it consumes 205 to 210 watts, depending on fan activity. That's 4.75kWh per day, or 142.5kWh per 30 day month. It produces around 0.2 BTC per day, and since I pay the equivalent of US 25c per kWh it costs me $1.19 to get $2.72 of income. $1.53 per day. Then take out income tax, which few people add to the calculations as they think they'll never get caught when miscellaneous USD$ appears in their accounts, and it's close to a dollar per day. $30 per month on hardware that today costs about $350. It's difficult to get excited.
If I could unlock the shaders or overclock further the calculation would be slightly more favourable, but power consumption goes up with hashing speed too. Adding a second card to the PC is not that efficient as you gain by not doubling up on power consumption by the RAM/mobo/PSU, but you pay the penalty of the CPU now running at 100% due to dodgy OpenCL drivers.
