yup, I’m in MN and the replacement cost of my arena is 400K. It would be cheaper for just the arena part, but still in that range.
MN requires better roof support than other areas given the snow loads we need to support, so costs may be cheaper elsewhere
I’m not sure exactly where the OP is, but 300 is near the max you get for pasture boarding in many areas in MN unless they are really close to the Twin cities or have some really great draw (like access to a park with riding trails). Mine have designated stalls they can use (and I will keep them in if the weather dictates). I have indoor and outdoor arenas and an enclosed bridle path. I provide hay and grain (with upper limit in grain, but I really don’t have anyone who hits that since I feed really good hay free choice.). I do have a 31 year old boarder whose owner provides some grain, but that was her choice coming in. I feed supplements, blanket, hold for farrier and provide other services for free (my boarders do not abuse this). I could probably raise my rates and the boarders would stay, but I have pretty loyal boarders (and only a few select ones).
Given the price of hay around here this year, I think most owners would understand an increase. The question is are they going to find a similar place for less money if they decide to move. When I started this place, people told me I was more money that others down the road. I said, “that’s fine, I understand the need to budget but this is what I need and if you want what I have to offer this is the price, you can check around and see what you get”. Some found a cheaper place and I ended up with boarders who appreciated what I bring to the table.
So for the OP, I’d check prices in the area, then figure out how much more to add. Then give your boarders sufficient notice.