Okay... you will save $100 per reflector, but you will also see a huge drop in output. Not that Im being critical, but in this case, you are comparing the best reflector out there to perhaps the worst (at least a lesser one).
As for cheaper... dont see how... not for a dual HQI system. The M80 ballasts are what they are. Sure, you might save $30 or soemthing on a dual M80 ballast, but thats about it. I have already listed the bargain basement price for a bulb as well. The only ways to go cheaper are with SE bulbs and non-HQI ballasts... but I really wouldnt do that if I were you. DE/HQI bulbs are the way to go... longer life, higher efficiency, better color stability, etc.
A T5 system would be cheaper, an FWIW, less wattage with more light (more useful light due to the spread). A 6x54watt powermodul system by ATI is $719, but thats the upper extreme... most other alternatives can be had for a fraction of that. A 6x54wattT5 retrofit for example a Tek retrofit from reefgeek is $449 with the bulbs. Its only 324 watts, but closer to the water, and more even output than the halide... you can easily get the same light levels in the T5 tank as a halide one. I used to favor halide only systems, but no more. The best is a Mh+T5 combo, but on smaller tanks than 100g, its just too expensive or cramped to run an effective combo system... so 6x54wattT5 is what i would suggest for a 75g. The advantage is that you can mix and match bulbs to tweak the color to be just how you want it... with halides... you have to pick a bulb and thats all you get.
