If you can do more than one load a day, I would get him to a 70/30 deal and a 30/70 deal on the next one. That way at least you don't lose money and have a good chance at picking up the client.

Are you breaking the mowers down at his shop? How well are you breaking them down? How much much time does it take to break down a single mower to the point you want it? Are you paying him different prices for steel, alum, stainless, batteries, spark plugs, ect? How much scrap does he generate in a week and in a month? Will he consider taking a base price per mower? Those are a few questions you'll want to have answered if you take on this job.