I have four, two of whom are insulin resistant, so my answer is "more than even I really want to know" and, they are why I do all my shopping at WalMart and Tractor Supply"
That being said:
I buy hay by the season and that cost me $1,665 for this season. I have 22 acres but, with two IR horses, I go thru a bale of hay a day anyway.
Maybe $150 annually on pelleted rice bran - no one gets grain.
That means I need a vit/min supplement. I feed EquiPride but I buy the 50# bags because I need to measure to the inth, what everyone eats. I have to drive 92 miles one way to get it, so I buy a year's worth, get a discount and keep it in the house. That cost me $920 a month ago.
Maybe $440 annually on Omega-3 Horseshine.
Then there's the arthritis meds and also the herbal supplements for the two insulin resistant horses.
Remember, I am feeding four horses and they all eat the same base
I do my own trimming, so it's the cost of trimming tools as-needed.
Vet bills are minimal - maybe $300 annually - the dogs cost more than horses. Divide just the base items by 4 horses and that comes out to about $868.75 per horse annually. Again, they are on my own property. There is additional cost for the arthritis meds and herbs for the IR; neither of which I ever planned on - especially two horses becoming insulin resistant.
I don't have a clue if that's cheap, expensive or in the middle - I don't ever think about it, I just know I have to give them the best care I can until it's time to send each one on to meet their ancestors. They are not commodities.
Then there's all the ancillary meds I have on hand for the normal boo-boos and, the boo-boos that many folks call a vet for but I don't.
Unless an artery is punctured, they need sutured, have serious tendon injuries that need x-rayed are involved, or someone needs the chiropractor, I take care of whatever goes wrong - mainly because I have been at this my entire adult life and half my childhood
My horses have been with me 20+ years, 18 years, 13-1/2 yrs, and 6 years. Horses are a huge commitment, so don't buy more than you can afford to give the very best care to, even in the worst of times;egardless of whether they are on your property or at a boarding facility
And ditto everything "Ferhoodled" said:)