I have, and Im sure many others have, knocked off 4 to 5 acres in less than 2 man hours. I have a buddy who is a LCO, and his 2 man crew mows 30 acres in 16 man hours! Thats w/a 60 and a 61 deck.
13 acres is definately feasible w/a 60z for one guy! Should take about 7 hours. Especially if its open, and relatively flat. 7 hours of mowing is about 7 gallons of gas. Every 3rd mowing the blades will need sharpened. Every 7th mowing an oil change. Figure these costs into the job.
You can cut the guy a break because you have 2 of his properties, and a buge one at that. Honestly, I would tell bim upfront that he is getting a multi property discount, and as soon as that other one sells, his price would vo up 10% since he no longer has multi properties. Figure out what you need to make hourly, how long you think it will take, and there is your price. You dont have to "give him" a discount, but if you tell him you are, then he may be that much happier. Tell the customer what he wants to hear,its part of sales.
Snethercutt, if my guy took 4 hours to mow 2 1/2 acres, I would fire him on the spot.