Your local tack shops will have bulletin boards - post a notice and check back regularly to make sure it can be seen. It's hard to describe, but I'm sure you've seen notices where they have their name and phone number at the bottom of the notice, typed sideways, and cut so people can just pull it off and take it with them.
What's a fair price? It's hard to say without knowing what you have to offer besides the arena. Do you have a tack room or spot for boarders to lock things up? Do you have indoor or outdoor shower stalls? How much turnout do you have? In my area, sw ohio, $175 for pasture care, and $200 for self-care is the norm, and it goes up according to the care and amenities offered by the facility. I am now paying $425 for full care, but the facility has more to offer, such as lots of trails. The facility next door also charges $425, but is a lesson barn and the students take priority over the boarders!
One other idea - you can rent out your arena on a daily basis. People with horses and no arena can trailer in, and have no problem paying to use one.
Insurance can be a deterrant to a lot of your plans these days, especially if you have a show. We've gotten around that by having "pretend" shows~
Also, be very choosy when taking on boarders - you are opening your home to strangers, and you'll be dealing with them daily. If I were a barn owner these days, I'd ask for references! I'm sure you have a contract drawn up, but try to think of every possible problem that could arise and address that in the contract. Even then, should a boarder dump a horse on you, it's very difficult to sell it without their signature, so you want try to avoid that situation.
Good luck! Give it time, and eventually you'll be having growing pains and need more room