Let's just say you had the MH up and running and your only cost was bulb replacement. For the sake of argument or laziness, let's say that you only changed those bulbs once a year. At the same time, you want to compare how much it would cost to crank out a DIY led setup. Let's just say for kicks and giggles, it costs you $750 (double Tony's figures above) for all the parts necessary to set it up.
MH setup (2 bulbs) = 150/yr.
Bulb replacement = yearly
LED setup = $750 (72 led array)
LED life = 50,000 hours
Run time = 10 hours daily (13.7 years)
You'll break even within 5 years. This doesn't even take into account the cost savings in electricity.