I have three of my own that live on my property so bear in mind this cost breakdown is probably going to be a bit different than most people's.
Feed and supplements- I'd say probably $200 a month.
Hay- I get a load of about 100 bales at $4-7 each every few months, so we'll say $100 a month on the cheaper end.
Seed, fertilizer, etc: $30/month
Tack and equipment: fluctuates a lot. $20-200 a month depending on what they broke in any given week.
Farrier: my horses are on a 6 week schedule, so that breaks down to about $25 per horse per month.
Vet: my horses get seen about every 4 months, so we'll say about $50 a month, more if it's around teeth floating time.