Well the least expensive thing about a horse is buying one.
We live on the mountain and although our horses have 12 acres to run on, its wooded and little grazing to speak of.
We pay [depending on supply available] 1,100 to 1,300 for a retriever landed at our barn of very good hay. We buy three retrievers to feed for a year of alfalfa and grass for 3-4 horses. Buying by the bale at the feed store [no rounds hereabouts] running 18.00 each, figure depending on bale pack [10-13 flakes] and flake weight, feeding twice a day a bale will last 3-5 days.
Standard shoes every 6 weeks 95.00 per horse. Specialty shoes are more and barefoot runs around 45.00
Vet standard twice a year is dental work and vaccs, plus a lot of states require you to pull Coggens. If you can give your own shots you can buy most over the counter except for rabies and WNV. Last year paid 1,060 in standard care. Thats not counting emergency calls for colic, lameness, bad cuts etc. Plus you'll need emergency supplies your self to have on hand....
We write our horses off as we are with the Sheriff's Posse, but on food and vet for three horses here at our ranch $5,504 last year with no emergencies.... thats 1,800 a year for standard everyday care and roughly 600.00 a year for farrier each horse - say 2,500 a year or 209.00 a month per horse....
Of course it doesn't stop there.. you'll need to add
Shelter a simple run in around 1,000 for materials and you build it to 20,000 or more for a new barn
Then you have to buy equipment - saddles, bridles, brushes, blankets, halters and leads etc., etc.....
Of course then you'll need a way to get them around so throw in a trailer and a suitable truck to tow it with lol
What kind of horse are you looking for? It is a buyers market now for horses, you should be able to find a very nice mount for very little money.