At my small, middle class, suburban school, head coaches get 6% of their teaching salary, so at a 40k base salary you'd make about same as you do now, $2,400. Track assistants get 3-4% (depending on how many we have) so $1,200-1,600.
My cross country program has around 30 kids with only the head coach getting paid through the school. We have to fundraise on our own to pay assistant coaches, otherwise it comes out of my pocket. My predecessor was a generous guy who didn't care at all about the money. He set a difficult precedent by paying his assistants out of his own pocket and buying his own supplies, rather than getting help from the athletic department.
I would coach for free too (and I have before) but for a young teacher, it's much easier to pay the bills during track and cross country season.