I try not to think of it too often
But here's my breakdown for one competition horse that is boarded
Farrier - $90 every 6 weeks, as he has front shoes (so we'll say about $650 for the year)
Vet - he gets his coggins and vaccinations, it works out to be about $200 in total for the year. Teeth are about another $100
Board is $425 a month so $5100 a year
Monthly Feed he has his hay included, but I give him extra, and it works out to be about $30 a month, so $360
Tack - is generally a one time expence, so don't include that for your list.
Insurance - I pay $200 a year for him
Trailer - same as tack
Hay - included in board
Blankets/Sheets - hah, yes, well I have a little obsession with that. He generally gets at least one new blanket a year, so budget $200
Bedding - he's not inside.
So, for him to be boarded in pasture, and assuming nothing bad happens, I'm looking at about $7000 a year to keep him.
That's depressing *sigh/