It's very hard to put a number on the cost of ownership for a horse. There's too many factors such as does your horse need shoes? Those get done a few times a year by a professional. Does it require a special diet? Does it have bad allergies? I hope not cause man it just got expensive lol.
Also a lot of people don't have land/place to keep their horse so they have to board it somewhere. Basically renting a stall for their horse to live in and typically a place to ride their horse such as a outdoor and if you're lucky, a indoor area so you can ride year round rain/snow/sun. This becomes extremally expensive. My girlfriend has a a cheap place for her horse and it's almost $500/month, but that's including a indoor area to ride. More than my truck lease!
I work for a couple fire depts and it's the same deal with equipment or clothing. It's such a specialty thing the price is automatically double what it should be I swear. Just because it's "safety" or "horse" related, it's going to cost more.
Don't get me started on horse trailers..... another reason why it'd be cheaper for a large animal vet to come and preform the procedure.