There is one business principal for pricing and it is this: The buyer decides for themselves what a thing is worth to them. The buyers collectively set the market price, regardless of cost.
First, find out the market rate. Find out if that rate is bringing enough buyers to leave room in the market for you, too (it depends on how much demand is out there for it).
Then, figure out if your costs make it profitable or worthwhile for you to be in the business. If you need that to include the mortgage payment, insurance and so on, include it. Because it is important to your world.
Then make a go-ahead decision, or a no-go decision. Or, a continuation decision.
Pricing at cost-plus-profit is a huge error that puts people out of business if it doesn’t match the market. If a barn is a high-cost provider and charges accordingly, they may not have much business because boarders can go elsewhere and pay less.
Buyers usually care less about amenities and more about total cost. That isn’t always true, especially in higher-end markets. But in general buyers are likely to choose to pay less for an uncovered arena, even if they would prefer a covered arena. Same for footing, turnout and other amenities. So another dimension is figuring out what buyers consider to be essential services, and what they do not see as essential services they will pay extra to have.
Sometimes it is tough to find the market rate if there isn’t much that is comparable in the area. That may mean you are offering a needed service without much competition, or that what you are offering isn’t that needed, so no one else is offering it.
It is also important to figure out how much available stall board in your area is actually being used. If a board barn is has many available stalls but wish they were full, that’s a problem. Either they are charging too much, or else there isn’t enough demand – maybe both. It will be hard to charge the same rate and not have the same problem.