No one can give you remotely accurate costs until they know a little more about your situation.
Agistment/board/livery can range from free to over $1000 a month, depending on where you live and the services offered. Doing it yourself can be cheaper, but if you can only source expensive feed and spend lots in travel costs, full board can be cheaper.
Again, feed costs vary. It can range from $1 a day for vitamin and mineral supplements for a horse on a great pasture, to $20 a day for a horse that requires heaps of hay and feed. It will also depend on where you live and the costs of feed.
Buying the horse itself - if its your first horse budget $1500-$3000. Additionally budget $300 for travel, $1000 for tack, $300 for rugs if you live somewhere that requires them. The tack cost can vary, but its worth getting a good saddle, or a good second hand saddle, rather than a cheap unbranded on.
On top of that there is farrier, vet and dentist costs. I also spend more than I spend in feed in petrol just getting to the horse.