WAG* between 500 and 5000 - it depends on a lot of things.
What part of the county are you in? Will you be boarding or keeping it at home? Trail riding, doing any showing, 4H, etc?
It will nearly always take more money than you thought - especially if you have an emergency vet call or two, those add up fast! If you keep it at home to save money, you will save some (after the initial outlay for fencing, shelter and storage for hay and supplies), but you'll spend lots more time in exchange for saving that money.
* - wild arse guess
I have no clue but I think that is a great answer