I wouldn't bid it by the acre myself. I would probably estimate the amount of time I was going to be at each park and give a total amount per park. With 3 mowers running you should be in and out in an hour or so, sounds pretty easy. I'd figure what you wanted per hour and submit it that way. Don't forget to look up though, might be time consuming when the leaves start falling if there are many trees.
How close are the ten parks to eachother? Figure that and the leaves, like Homer said, add the estimated manhours(should be around 3 per site) then add fixed overhead and profits. All that should come up to around $150 per site per visit???