I might be a bit rusty, but here goes....
Food and board is a major expense. Also you need worming a couple times a year, plus regular vaccines and sometimes testing depending on what you want to do. Some areas requires a clear test for things before you can transport a horse.
The equipment cost is actually minimal because it's all a one time purchase(except small things). Buying halters, lead ropes, hoof picks, brushes, etc, should only need to be done once, then items replaced as needed.
Horses need regular vet care, and they are renowned for managing to injure themselves. Having a large animal vet called out because your horse found the only expose nail to slice an eyelid half off means a rather large vet bill(yep, had a horse cut the eyelid almost OFF somehow), and they need hoof care on a regular basis. Depending again on what use you will be making of the horse will determine whether your horse requires shoes or not. Trimming hooves is cheaper than dealing with shoes, but some horses will need shoes even for just the trail ride weekends, while others can do without even with daily work.
The time invested is considerable. The danger involved in working with horses is also considerable. They are a large rather unpredictable beast capable of kicking a human skull in on a whim. You may or may not have to have some sort of insurance, depending on where you are keeping it and what use you make of it.
That's all I can come up with right now, hope it's useful.