First of all, I commend you for being SO prepared, having leased and done your homework before taking on such a big responsibility, so bravo!
Is the horse shod, or barefoot? Mine is barefoot and I spend probably only $260 per year on her trims ($40 every 8 weeks), but I know shoes are a whole lot more than that! So for me it comes out to about $20/month for trims.
Then for vet stuff, barring any unexpected emergencies (which ALWAYS crop up one way or another!), probably another $400 every year on routine vaccines. So that's another $35/mo if you average it out, so now we're up to $55 for me.
Getting her teeth floated is another $200 and we get that done once a year, but I know some whose horses need it twice a year, so that brings my monthly total to around $70 so far.
If my worming was included in board, then I wouldn't add that in but since it's not, that's probably another $10/month or so, depending on what kind of wormer I get. So we're up to $80 now.
Equipment doesn't really need replaced often if you take care of it and it's good quality, so I probably wouldn't work that into my budget. But as for showing, I'd need to know what kind you plan on doing, that can get expensive! In my area, a recognized Horse Trial (Eventing) would run me around $150 to enter, then pay for a stall on top of that ($40/night), and gas to trailer out there would likely be another $50, so 1 show may cost me $240 PLUS then I'd need to purchase hay to bring along so let's say another $10 on top of that (hay is only $5/bale right now in my area WOOHOO!)
So anyway, you can see how things might add up...but for regular maintenance of my barefoot pony with NO extra vet visits for emergencies, I spend about $80/month in addition to board, NOT counting anything I do for shows.