It depends upon what area of the country you are in and in what style you want to keep them.
I've got four horses, rent the stable they are housed at, and they do some showing -- and I live in the Midwest. The basics for me comes out to roughly $150/month/horse. My horses are stabled with about 5-6 hours turnout during the worst of winter (in the summer they're out most of the day), get wormed four times a year (6 if they're traveling to other places), routine vaccinations 2xyear, dentist work once a year, farrier is out every 6 - 8 weeks, and they get alfalfa/grass mix hay, a grain mix for horses doing pleasure/some performance work, and a broad spectrum vitamin/mineral/salt-electrolytes/probiotic/Omega fats/joint-tendon supplement mix. Then there are the added costs of any emergencies, show fees, training fees, buying stuff for the horse, fixing stuff the horses break, mowing the pastures, seeding/fertilizing the pastures, cleanup expenditures. It adds up.
It can be done more cheaply, but you're still looking at at least $90/month in this area. Depending on your area and circumstances, it might be cheaper still or it might be more expensive.
Most full care boarding barns in my area that do a good job are charging around $250 - 350, and they don't provide feed supplements, farrier, or vet care.
The best thing to do is get in contact with local horse professionals (vets, farriers, boarding stables, trainers, breeders) and ask them what the costs for keeping a horse (and what that entails) in your area and situation are.
Watch out -- horses are addictive!