If the actual power draw was consistent with the lamps' rated wattages yes. Unfortunately, it gets a bit more complicated than that because not all ballast/lamp combinations will draw the same power.
In other words, just because you're running 250 watt lamps doesn't mean the ballast is drawing 250 watts from your wall outlet.
Right, and it might be easiest to just divide your total electric bill ($) by usage (KWH) to find your actual cost per KWH. The way some utility companies break up the costs on the bill showing separate delivery costs, cost per KWH, etc. can be a bit confusing...