I'm not too sure about a 400W bulb, but if you're going to be running that much light, you will need some sort of fan or cooling system as they do get very hot.

I just run a small 150hps here, and it costs me maybe $10 extra a month. So with a 400hps with some fans, about $40 extra a month, maybe less. Not a huge noticeable diff., so it's nothing to worry over.