I live in the Sacramento area. I have a 12 year old trail horse. I keep her at a full care barn that has two arenas (one covered), roundpen, trail access, feeds supplements, hay twice daily, takes flymasks on and off, stall at night and in bad weather (actually she is stalled midday until evening this time of year due to the heat, and out all night and in the morning). Board is $355/month.
Now, with all other added stuff (supplements, vet bills, shoes/farrier every 6-8 weeks, sand lax, meds, grooming stuff, etc. etc.) I have calculated it is averaging an additional $150 dollars a month which makes my total expenditure about $500/month.
There are so many variables though. When I was in college I kept my gelding at a place where his hay and board were free because I cared for all the horses on the property, I trimmed and cared for his feet myself and did as many of his shots as I could (I actually still do that). Anyways, my monthly bills averaged $50/month. So that is the other end of my experience cost wise.
So it all depends on so many factors. Are you boarding? Does your horse wear shoes? Are you able or wanting to work at a facility to keep bills down? Are you wanting to show? Do you need a trainer? Lessons? It goes on and on. Everyone has a different situation, but hopefully the above gives you some ideas.