I own three and keep them on about 4 acres so I pay no boarding.
I keep all my receipts and tape them to a calendar.
I posted a spreadsheet a couple of years ago, but just the cost of ownership is about $2000 per year for each horse or roughly $5.25 per day, or, in my case, $17 a trail mile.
That cost doesn't include travel or expenses related to the trail rides, just the cost of keeping them.
If you had more land I think this could be done a little cheaper as I stall my horses 8 to 12 hours a day. Shavings are a significant part of that cost.
Here's a summary for the year 2014:
Apple treats 214.87
hoof trims 960
Rolls of Hay 600
fly spray 359.9
month per horse
day per horse 5.49
For we wrestle not against flesh and blood, but against principalities, against powers, against the rulers of the darkness of this world, against spiritual wickedness in high places.