I can think of a few places that are all-you-can-eat (tabehodai) for 1000 yen. I wouldn't bet on the quality of food, but sometimes you luck out. 500 is insanely cheap, but it is near the break-even point (if you aren't including rent)
So you are really looking at 250 yen; Around half the break-even point by your estimates.
Groupon takes a commission which varies but is reportedly around 50%.
So the client business gets 250-yen per coupon.