By the Job.
Had an old man that would always as me to do extra stuff after I mowed. He would ask how much to trim his bushes. 4 boxwood's about 3' x 3'. First time I said about 20 bucks an hour. He made a big deal of it, wanted to know exactly how much. Told him 10 bucks a bush. Done deal. 15 minutes later I was pulling down the street with extra 40 $$. What would of cost him about 10 bucks cost him 40 because of his wanting to know the exact price.
Sometimes you win, sometimes you lose, and sometimes you get rained out.