I was running 3x250 MH and 4x 80w T5's with eballasts. When i went to LED I saved about $50 a month in power, the cost of halide and T5 bulbs but biggest savings came from AC. I live in a 2200 sq ft house, with vaulted ceilings in central Florida where most of the year the AC runs and most summer months the 5 ton unit struggles to keep the house <80 when set on 74. Hard to pinpoint but I would estimate my bill went down $150 a month in peak summer. Of course given my LED's died off I'm back to that MH fixture for at least a couple more weeks when I get new lights. The failure of my Kessils negates a lot of cost savings but you can't discount the aesthetics along with heat and power.
Probably my biggest fear right there. I have 2 AP700's over a 6ft 30" deep tank and know I'll likely need to supplement with T5's at some point. If the Kessil's, which are 2 years old now, fail before 5-6 years old then I made the wrong decision most likely.
My issue is I'm also in central Florida and my tank is in my office. 3 MH's, even 250W fixtures, running 8+ hours in my office is begging for my it to be uninhabitable lol!