U
Ultra Current
If you are taking wild-ass guesses at everything and adding it up, you'll never be able to afford to do the installation.
Regarding running 120 volt ballasts on a 100 amp service, you are incorrect. As Iron Lion already pointed pointed out, the panel would be fed with a 100 amp double-pole breaker. This means that you actually would have (2) 100 amp 120 volt circuits in the sub. You would just need to insure that the load was balanced, and it would be fine.
Any ballast that has 20% ballast losses has something wrong with it. I've worked on literally 100's of HID lights, and I don't recall ever seeing one with more than about half of that.
If a 10 light grower can't pay the extra $100 charge then that's very funny. Listen to what you are saying. You say that he can get there with 120 Volts by using 2 x 100 amp breakers. That's 200 amps and the user is trying to only run 100 amps. And every ballast uses more than 1000 watts so don't try to mislead people with your slick word play.