I have two retired geldings that are kept at my parent's house. They have a large pasture and do great on just pasture in the summer. In the winter, we feed hay and that is free. We have our own land and "rent" out much of it to a neighboring farmer. In exchange, he makes my SIL's hay pasture and gives us the hay. (SIL also has a horse at my parents' place).
Costs are annual vaccinations (a very minimal amount, since I give them myself), worming (I generally alternate the "cheap stuff" with more expensive wormer) and farrier visits. My horses are generally done every 8-10 weeks and the cost is $30 per horse. I used to do my own farrier work but am getting old and lazy.
Teeth check/floating is a couple hundred dollars per year, including the farm call since we don't have a trailer to haul them to the vet.
My 22 year old QH is getting stiff in the rear so I will have to start giving supplements. (Provided it doesn't turn out to be some other issue).
This is all in a "normal" year. If one or more gets hurt, costs skyrocket.
I'm getting two ponies: a weanling and a yearling. The vet's going to be coming out to geld both of them, check out my QH's lameness, give vaccinations (since the vet will already be out I'm just having her do it), plus teeth check and floating on my two older geldings. I don't even want to THINK of how high that particular vet bill is going to be.
I have one other horse and he's kept at a boarding stable. Board is $275 per month (box stall board with regular turnout). Worming is included but farrier work is not. I'm thinking he's going to need front shoes so that's going to be expensive. He's very green broke so I'm going to be paying my trainer to work with me and him. I'll probably get a "half time training package" ($360 for 3x per week or $260 for 2x per week). Hay is included but grain is extra so if he ends up needing grain that'll be an extra cost. I'm going to estimate around $600 per month.