No idea about tracks on farms, or costs, but our local council made a children's playground area in a muddy part of the park - put down a load of hardcore, then very small gravel, then that black holey stuff that grass can grow through, so kids don't get hurt if they fall. That was about 10 years ago I think - just as I moved here, and it just looks like the rest of the grass in the park and they mow it same as the rest in summer