I'm using the cheapest lighting in the universe on my 75.
It consists of three twin tube fluorescent shoplights (purchased at Lowes and Walmart) for about $8 each. These fixtures hold a total of 6 bulbs, 240 total watts of Normal Output light. My bulbs are two actinic, two 10,000K, and a couple of 6,500K daylight bulbs (the 6,500's can be purchased cheaply at Lowes too).
I built my own hood and installed the shoplights in it -- with bulbs the total cost was probably something like $125
I can keep a lot of different corals with this light, but not anything with really high lighting needs (like many acropora and most clams -- although I do have a few acros and a Derasa clam).
I don't really recommend this as the best way to go. If you can afford a little more, look into getting some VHO's, PC's or T-5's -- personally I'd check out the T-5's. MH is superior lighting overall, but also has a $uperior price.
I'm including a pic of my two year old 75g tank, just to let you see what's possible with this lighting
HTH,
Mariner