ReactiveLama, all I can tell you is around 60-80W of LED's should be able to replace the 400W HPS and your electricity saving will be a least 70% on that bulb, not to mention there's no need for a cooling solution.
You can figure out the cost of the electricity easily enough, however set up costs I don't have a clue. I'm in China and so have no idea how much you have to pay for LED's.
However if you were will to build it yourself that would be much cheaper.
As for the yield, make sure the wavelength of the lights you chose are correct, and wait a while, to see the results of the other guys on here testing.
If you went watt for watt 400W LED vs 400W HPS, LED would win, however 400W LED would be pretty expensive.
Also someone mentioned on here that UV was really important, LEDs give off hardly any UV light.
