I don't know how much all the what-ifs and variables matter, given that this is just a daydream estimate as I understand it
For those purposes, I'd suggest your
lowest likely cost, assuming the horse lives out 24/7 on 2 decent acres with safe strong fences and a safe strong run-in shed, would look something in the ballpark of this:
$30/month farrier and worming, x 12 months = $360/yr
$150/yr vet check and vaccinations = $150/yr
5 months of hay @25 lbs/day = 3750 lbs, perhaps ~$350/yr (ish!)
$150 dentistry every 2 yrs =$ 75/yr
$90 waterproof turnout sheet, replaced every 3 yrs=$30/yr
______
scheduled yearly expenses = roughly $1100
PLUS ON TOP OF THAT there will be repairs needed to fences and shed (unpredictable amounts and timing) and unexpected vet bills ranging from none to whopping (I would recommend at least $1500 cash in the bank to 'self-insure' against this).
Also some horses will be more expensive to keep, e.g. ones with ongoing feet or medical issues that have to be managed or require you to have more-expensive farrier or facilities than a 'plain vanilla' horse would; and if your fences are unsafe, your land harbors hidden hazards like metal trash, or your pasture is insufficient in quality/area to gracefully support the amount of grazing it gets, costs can rise (sometimes *substantially*) from the above.
However you will not want to keep just one horse. Two, at least. They really do MUCH MUCH better that way, are easier to deal with, damage your fences and escape less, are healthier and happier, etc. And of course for two horses you would need twice as much land (or more) to avoid them ruining the soil and degrading the pasture into a mudpit and requiring to be fed a lot more hay per year.
HTH,
Pat