You cannot possibly assume that any horse you buy would be fine and dandy on pasture and barefoot.
Don't get me wrong, I have absolutely no problem with pasture board and barefoot horses - I like both, and have pasture boarded in the past (though I don't anymore, my horses are on pasture a good chunk of their day). I currently have one mare who is barefoot, and one who simply can't go barefoot so she has to have all 4s.
Not every horse is suited to pasture only board, just as not every horse is suited to barefoot.
Also, are you sure the places you will be boarding will supply all feed? What about in winter if the horse requires grain or BP, or if the horse requires various other supplements for other reasons? I doubt pasture board would supply that. It's hard around here to find a place that will if just pasture board.
But, since you asked, this is what I pay annually for the basics for one horse (though I own two).
Board - $440 per month for semi-self / $5280 per year, I used to full board both my horses, and that came to $13,500 total for the year
Vaccinations - I do myself, and I think that comes to around $200 per year, If I had the vet out it would probably be around $500 for the year
Teeth floating - $350 once per year, though my other mare needs it 2-3 times per year so that winds up being $1050, and usually a bit more since she's difficult to handle for floating
Worming - Probably $100 per year
Farrier - $170 every 5.5 weeks / just over $1600 per year
Bedding - $300 every 3 months, and usually another $100 worth to tide me over until the next shipment arrives, so probably somewhere around $1600 per year
Supplements - maybe $500 per year, I've been extremely fortunate where my BO supplies most of my supplements for me, but I do have a couple extras I get
Emergencies - vet charges $110 call out fee, and I usually have an average of 2 emergencies per year it seems, and those have ranged me anywhere from $500-$2500 up front
Misc. - this includes any blankets that need replacing, or any saddles/tack that need replacing (not things I WANT, but things that are NEEDED). I've spent anywhere from $300-maybe $5000 at most in a year on these sorts of things
So for one horse it costs me anywhere from $12,600-$18,000 for my basics only, and that was obviously quite a bit more when I did full board - this doesn't include things I simply "want" (nice saddle pad, new breeches if something catches my eye, etc etc.) also doesn't include showing fees.