Routine -- assuming you have full board including feed and hay. This is what I do for Solo.
--trimming/shoeing (every 5-6 weeks)
--worming (every other month)
--vaccinations (2x per year)
--Coggins test (once a year)
--teeth - I only have to get Solo's done every other year by the vet
--chiropractor (every 3-4 months -- good to get a working horse at least checked out once to see if adjustments needed)
--supplements (hoof and joint -- I get a pretty good deal through SmartPak, love 'em)
--general supplies like fly spray, treats, vetwrap, SWAT (continual expense, varies seasonally)
--online shopping at eBay and evil horse catalogs (way too often)
I think that's the basics. Just be prepared for a shock -- both sticker shock and the shock of the overwhelming responsibility. I've been riding and taking care of horses since I was a little bitty kid, but I STILL didn't realize what a different experience ownership is. I would say I spend at LEAST another $200 per month and that is probably on the low end, ON TOP OF board (which does include feed, hay) just to keep Solo in supplements and the other random stuff you always seem to need to buy.

Solaris -- 16 hh Appendix Quarter Horse = MY DREAM COME TRUE!