400 watts will only raise the electic bill about $15 a month tops. You can probably cut nearly that much by reducing the temp on your water heater to 120 degrees, switching to cfl's, turning the temp down on your termo or up on your ac.
I would go with 1000 watts. You will be disappointed if you don't. You can never have too much light but often people get cheap and suffer from too little light. Remember if you are using for flowering only you will be running on 12/12. So it won't be a constant power drain.