With a pair of 175w MHs my tank costs about $25/month in electricity to run (that's the entire tank, incl. pumps, skimmer, heater, cooling fans, etc.), but like muddybluewater said, unless you're going to use actinic supplementation you won't see a big change going from 324w of t5 to 400w MH, at most 100w difference when you figure in ballast inefficency, which could be offset by changing out 2 60w incandescent lamps in your house with 13w CF lamps if you want to look at it that way.