My friend works for the power company and I asked him the same question. A watt is a watt, no matter what. The power company bills you in watts per hour.
The largest PC
bulb (in an aquarium fixture) that I have seen is a 130w.
Lets say you had a fixture that had 4 130w 36" lamps, thats 520w. Now consider that in the same space, you can have 4x250MH bulbs, or even 4x400MH bulbs. Thats a lot more power, hence a bigger cost.
Also, if you have noticed, most of the higher watt PC
's are longer. I have been looking for a 24" PC
bulb that is more than 65W. The increase in watts produced by the PC
is, I think, a function of how long the buld is. That is to say, a 130w 48" bulb isn't really brighter that a 65w 24" bulb, it just covers a larger area. Whereas, you can get MH
bulbs in 150; 175; 250; 400w for the most part, are all the same size.
My point being, the cost to run MH
's increase beacuse you can increase the amount of light power for any given space.
hope this make sence.