How long is a peice of string?? I heard a saying once- "poverty is owning a horse". And it rings true. They are soooo rewarding though, and I endure poverty at times in order to keep my girl.
To get a good idea of how much it will cost you to buy and keep a horse, I would suggest some work at a riding school or stud. They usually love voluntary help, and are happy for you to come down once a week and help out. This is a great way for you to get some "behind the scenes" experience, and you'll know whether horses really are for you or not, without the massive outlay of money first. It's also a good way to build contacts and friends with horsey experience, which I have found invaluable.
Buying the horse itself is only one piece of the huge pie that makes up owning a horse. You also have to consider-
Lessons- especially if you are inexperienced
Food- price will vary depending on the horse
Farrier- every 6-8wks without exception
Teeth- every 6mnths to a year
Vets bills- You have to have a bit saved up, because they don't like doing payment plans, and horses are generally a great deal more pricy than smaller pets
Wormers- every 6wks
Tack- bottomless pit of money required
Rugs- initial outlay and don't forget repairs.
These are the basics that you will need to be able to afford in able to keep a healthy horse in good condition. But, like I said, if you have a passion with horses and you can afford it, it is a very rewarding experience.
Good Luck :) Hope I haven't disheartened you, but I think it's important for you to know what you are getting yourself into .