3000$ - 5000$ per year. Plus initial costs like purchase price which will depend on type, age and health and training level of the horse. Like you could get a broken down elderly nag for maybe 500$ or maybe an unbroken wild mustang gor the same but if you want a horse thats in good health and trained for riding its going to cost several thousand dollar depending on breed. If you dont have it youll have to spend a couple more thousand on a transport trailer and several hundred to several thousand on saddle and tack. Yearly costs include food, vet bills, boarding or bedding, farrier, other gear and equipment, boarding or caregiver costs if you go on vacation. And thats just monetary costs. Time wise it has to be groomed daily, stall cleaned at least once and preferably twice a day so they dont stand/sleep in their own pee/poop and get hoof rot and illnesses. Yearly costs also dont include veterinary emergencies like colic or breach births or snake bites or equine flu, etc.
I'm not sure exactly how much it costs, but it costs a lot to have a horse. You have to pay for vet checkups, someone to deal with it's feet, equipment(bridle, saddle, etc.), you have to buy all of it's food, and you really need a LOT of space to have a horse. It should have lots of space that it can run around and a big stall.