Depends on the area and what amenities are offered.
It would be cheap in northern metro Detroit/suburban Flint area if it had an indoor arena, offered blanketing, jumps, heated observation room, washrack and nice tack rooms.
It would be expensive in rural northern Michigan no matter what the amenities.
I paid $300 a month for stall board at a place between Ann Arbor and Hillsdale, MI that was a 3 day event barn with a large outdoor, jump course, mats in stalls, trails, indoor arena, trailer parking, they gave my horse supplements and didn't mind my daughter coming out and playing in the round pen while I rode.
I currently pay $225 a month for pasture board with an indoor, nice tack room, supplements (I pay, they give), trailer parking, trails and state land nearby. Plus, if my horse gets injured or ill, they'll stable her at no extra cost until she recovers. They also don't mind if DD comes out since she and their son are the same age.