Not enough information! What do you mean by "going rate"? Is that full board, partial board, including blanketing or no? Training available or no? Jumps available for use? Wash rack? Tack lockers? Trailer parking available? How much turnout, are the fences safe? Outdoor arena? Trails? What kind of feed and hay?
Generally a barn/farm for rent is already going to come with an asking price per month. You start with that payment, do all the math including insurance, supplies needed, staff, farm equipment purchase or rental and add that to the rent. Then you will know how much you need to charge for board, lessons, training, etc.
Definitely do the math. I figure that I spend about $200 per month for hay and grain per horse. If you are keeping the horses in for 12 hours per day, add another $80 per month for shavings. Once you pay for routine repairs, insurance, an allotment to cover the expenses for boarders who don't pay their board, and someone to help you clean stalls, I don't see how you can make any amount of rent work. How are other barns covering their expenses and only charging $350 per month?