Up here, self care with really nothing for amenities, would be $50-75.00 per horse.
Shelter is not optional, so unless their are trees in the bigger section, or good hills, you will need to build a shelter. Is there anywhere to ride? Any tack storage? Hay storage?
I would go about this as first figuring out what your costs will be to have the horses on your property: Insurance, increased use of power, water (unless on a well), fence maintenance, increased wear and tear on your driveway, and so on.
Then look at places nearby and see what they charge to get a feel for the market. Make sure you compare apples to apples...so compare what you can offer to what they offer. Make sure this number is higher than your costs
Consider too, the risks/potential headaches:
1) Owners that don't feed/water. As property owner, you could be held accountable for horses that are neglected.
2) Disputes between boarders about someone stealing hay/not providing hay. Or really any other disputes
3) People showing up with the entire family and messing about in your hard all afternoon.
4) Horse being abandonned on your property.
5) Vet emergency with the owner nowhere to be found.
7) Boarders "borrowing" your stuff. Remember you are giving them access to your spigot, so they will have reason to be up close to your house.
Likely better off storing RVs.