My former instructor offered to let me work to pay for lessons, she paid at that time minimum wage, which was about$7 an hour back then, it's been several years back now. I was allowed to work off one lesson each week and then pay for the other. The work involved everything that you're doing and possibly more.
Does the barn have other paid employees? If so see if you can find out how much they make. Then see if you can get the BO/BM to write out a contract for you, that way they can't accuse you of not paying board if something goes south in the end.
The contract should state how much work- x number of hours each week, is required to pay off X amount of board. That way you can factor in how much you are being paid/ deducted from there. When on paper certain things look a little more reasonable or not depending on the amount of work expected.
If it does not seem reasonable ask about other options they might be willing to offer. My instructor would offer discounts to her students if they allowed her to use their horses for lessons. They could chose a reduction in board or a reduction in lesson prices in accordance with how much their horse was used that week.
You could also see if other boarders would help you out in some way, if you have the time and the skills, offer your hand at braiding/banding, clipping, grooming, et cetera for a fee. I know groomers around here make $40 or more for mane braids and $70+ for tails, clipping can go as high as $200 for a really good job. This could all be applied to your board fees.
To me it seems somewhat reasonable, but I do not know the full cost of your board and how much it reduces it by. If I knew those factors it would be easier to determine. Either way it is up to you to decide if it's worth continuing.