I do work in exchange for riding, and it's worked out pretty well, although there are always issues.
I feed on Saturday mornings, every week at 7 am. I feed roughly 30 horses hay, stablemix, or a mixture of these and grain, and 2 horses get their own completely custom mixes. Then I clean all of their waterers, and clean almost all of their stalls, take blankets off, so roughly 3 -4 hours of work depending on how fast I get it all done. The barn has a wild mustang, and several "problem" horses ranging from aggressive to spooky, to door darters, or that I have to turn out and then catch in order to clean their stalls. So I would rate my work at 10 dollars an hour, because a person doing this would have to have a fair amount of experience to handle some of these animals. So 40 dollars a week, or 160 dollars a month.
In exchange I get to ride this morgan gelding with a "mind of his own". No one else rides him, he's very hot and testy. It takes me a half hour of crap before I even get a decent ride out of him. And I maybe ride once a week, so the owner of the facility is getting a way better deal than I am, I can't get a lesson out of her if it kills me.
So just know that in many cases, this is what happens. You end up working your butt off for something that ends up not being a very even exchange. I like horses so I don't mind doing it, but it's not even close to an equal exchange.