the up front cost is less but on average water cooling is cost wise about 40 percent more efficient, not 70 percent, and when you add in the extra heat you are gaining from the lights, it drops it down to about 25 percent...
however, if you figure that for a large sized op, the ac you would need would probably be about 2000 watts, if you save 25 percent of that, its an extra 500 watts you can run in your room, which will in turn pay itself off in no time..
the main thing about the water cooling is its efficiency, and allows you to run more lights for the same money, and get the same cooling effects.. not to mention in a co2 enriched room, it means you need no outside venting, which will also save a ton of money on co2 or propane/natural gas.
ideally i would like to run about a 200 gallon res, with a 4 hp chiller, keep the water at 35 degrees, and run a system of hoses to 6-8 air cooled reflectors, with a 4 inch fan and an icebox on each one.. you could easily keep temps well in check, run a coil of hose in your res to keep that cool, and take care of the heat from a c02 generator..
all without needing any outside ventilation... sounds gravy to me