It depends on the size of the farm, how many markets a week the farmer goes to, how big the markets are (the bigger the market the more sales that will be made), how much food the farmer brings to market and how long they have been doing this for a living
A small farm that does one small market a week may net around $75 a week
A large farm (over 200 acres) that does 10 large markets a week may gross $1500 per market per week. But that farmer will have to pay employees along with other costs such as insurance, market fees, fuel and wear and tear on the market vehicle, etc., so ends up netting maybe $200 per market. But doing 10 markets a week means netting $2k. Though that person will work around 80 hours a week for that money and doesn't get vacation or health insurance (unless they buy it themselves). Oh and the $2K a week is for 25 weeks, not 52.
The bigger the market the more money one will make. The longer one has been marketing the more money they will make as it takes a couple of years to building a customer base to the point of profitability.Most farmers who do farmers markets. the more markets one goes to the more money one will make. But all of this is to a point because the more markets one does the more employees one must have to sell the produce and work the farm. So most market growers opt to do no more than 3 or 4 markets so they do not have to hire a lot of people to get the planting, harvesting and marketing done.
I know I found for every hour I spent at market 8 hours was spent on the farm planting, growing, maintaining, fertilizing, harvesting and post harvest handling of the crop before it got to market. I know I made a living doing FM's that was enough to live cheaply and be able to afford a mortgage but i was not getting rich doing it and it is a lot of very very hard work both the actual farm labor and the selling face to face for 5 hours on market days.
I farm and sold at farmers markets for over 15 years.