I'm not sure what would be your stall fees but a lot of the time it is extra for bedding, turn out, and storage for your hay and grain. There will be little things like treats and salt licks. Having their feet trimmed and shod is getting really pricey. I have to pay about $95 (per horse) every three months for trimming and shoes (I'm no longer a teen that can do it myself) for trail and endurance riding. Feed is probably the most costly- just because they are in a stall doesn't mean they eat less.
Vet checks (and you will need them in a boarding environment) cost for the vet to come out and what ever they have to do (sometime you can split the visit fee if other people have a same day appointment). I'm one of those people that believe you should be ready for the responsibility first. That means all the grooming, first aid equipment, and tack. You don't have to have a silver encrusted headset and saddle, but they are pricy.
Starting from the ground up is very costly but a rewarding experience. I have three quarter horse mares and I adore them to pieces. The one thing to remember when having large animals is that things happen. You need to be prepared to handle anything. I woke up this morning to my 16hand, 1400 lbs buckskin mare had a snout full of porcupine quills. Crap happens
Good luck and let us know what you get