It's really expensive. I used to have a quarter horse when I was younger and he was great, very gentle. My dad has a appaloosa mix, he's a little stubborn. I think you can get a saddle for around $250, think about spending another $100 or so on tack, then another $100+ on food. If you don't have a large enough area, they need a good amount of room, then you may have to rent a stable.
Here's a picture of me with Bubba Pal when I was around 12 or so, the quarter horse.


And here I am 10 years later with the Appaloosa mix