That varies a lot from one area to the next and even from one stable to the next in the same area. For example, I get $150 a month and you either take care of the feeding yourself or pay a fair share of the feed bill (I keep the receipts available) and I'll feed them. I also take care of minor grooming if the horse is well behaved. I also call in a farrier that I trust with my own horses every 6 to 8 weeks. When I get an appointment with him, I contact the owners and they have final say as to whether their horses get done.
There is a guy 2 miles down the road who charges $350 and you do your own feeding and everything else, all you get for your money is a place for the horse to sleep. But, he has a big closed stable and I just have a smallish 3 sided one.