Really? You want to go there? You didn't specify how much you pay for electricity. Without that VARIABLE, there is no way to calculate how much it will cost YOU. Also, you didn't specify how long you veg for. There's another VARIABLE that you left out. Did you even wonder why nobody had answered your question? Aside from the fact that it looks like a child wrote it, that is...
P.S. - I've never used a 400 watt MH in my life, as they're too inefficient for my tastes, so I have no idea what it costs to run one. My "girls" are perfectly happy sprouting/rooting under T8 tubes.
