Calculating power usage costs isn't guesswork- it's just a little bit of math.
Power is sold by the kilowatt-hour (kWh). A 1000 watt load running for one hour is 1kWh. A 2000 watt load running for 30 minutes is also 1kWh.
Look on your power bill for your per kWh price. It's probably somewhere between 8-12 cents per kWh.
If you run a 1000 watt light for 12 hours, it has used 12kWh. If your power cost is 10 cents per kWh, those 12 hours have cost you (10cents x 12 =) $1.20.
If you run a 400 watt (0.4kW) light for 12 hours, it has used (12 x 0.4 =) 4.8 kWh. If your power cost is 10 cents per kWh, those 12 hours have cost you (10cents x 4.8 =) 48 cents.
ngt is right; small CFLs as you find for replacing household incandescents will definitely grow plants- in fact they're quite nice for seedlings, clones and for slowly vegging mother plants. They'll flower too, better than any other fluoros, but they just don't quite make the density as does HID lighting. It's not only the number of lumens (apparent brightness measure) but level of light discharge intensity which make fat, dense buds.
There's some
really big new CFLs out there, up to 300W (replaces a 1500w incandescent in brightness), but CFL is still a low-intensity light source. Even a 300W CFL (or a ton of little CFLs) will not make the bud size and density of a similar power HPS due to the much higher light intensity discharge from HPS lighting. The big new CFLs are much better than any previous fluorescent for flowering, though!
If you're going to pay for the kilowatts one way or the other, you may as well use the light that produces the best quality buds. The best tool for the job for flowering cannabis remains HPS, though you can expect some interesting new developments in high-efficiency, high-output lighting in coming years.