Power drinks

Three men walked into a bar, each ordered a DB. 'That'll be twice the normal price' the bartender said. So the men quickly downed the beers, and pushed three empty glasses toward the bartender. 'Half price, I think you mean' said one of the men, as he put half the normal price on the counter. Having quenched their thirst, the men departed, but not before leaving a tip of 0.01029996 of the full price.