I keep track of all my purchases in my checkbook register, so I can go back over the course of a year and have a pretty accurate estimate what my costs are. Costs will vary per month so some months are more expensive than other months - vaccines only two months out of the year, some months you have the farrier out and some you don't, horses eat more hay in winter than summer, etc. And as others have said, costs vary widely by geographic region. You will probably get more accurate info asking this question of other people who live in your area.
For what it is worth, here is my breakdown for 2 horses being kept at home and me providing all the labor.
Grain and supplements - $990 per year
One of my horses is a hard keeper and eats a 50 pound bag in 2 weeks. The other is an easy keeper and eats 1/2 cup of grain per day. I provide a free choice mineral block.
Hay - $960 per year
For me, hay is the highest variable cost. Market prices change, horses eat more if the average winter temp is 10 rather than 30, etc.
Farrier - $1187 per year
One horse is barefoot, one horse needs front shoes due to poor front end conformation.
Vaccines - $108 per year
For my area, I do West Nile, Potomac Horse Fever, EEE/WEE/tetanus, and flu. I do these myself. Have to have the vet do rabies.
Dewormer - $96 per year
Vet care - $449 per year
I account for three farm calls per year - one for my rabies/Coggins and two for emergencies.
Dental care - $240 per year
Misc. supplies - $458 per year
In the summer I use fly spray while riding. I use Shosheen in tails because I hate them being tangled. I leave some money aside for repairs to my barn, fence, and pasture (fixing rotten posts, fuel for tractor for mowing, etc). Remember that if you plan to provide pasture as feed, you will need to account for pasture management costs. I don't have enough space for my pasture to provide actual feed so I don't account for it.
This makes my average monthly cost $374 for two horses.
When you are thinking about what your own costs will be, overestimate. I made the mistake of not overestimating when I returned home from college and immediately bought two horses. I knew what costs were for the area and I assumed I could get by doing the cheapest of everything. I could afford the two horses if I bought the cheapest hay, used the cheapest farrier, etc. but in reality you can't and shouldn't get the "cheapest" of everything. That decision ended up causing me a lot of stress, because I couldn't afford the standard of care I knew my horses needed.