Depends on if they are doing a full care or partial care lease, in my opinion. I pay for my lease geldings board. His owner pays for his farrier, supplements, vetting, etc. I still have full access to him. If you are going to be providing vetting, farrier, etc, I'd consider $200 a month. If they are going to be providing the above, I'd considering doing free or a maximum of $100 a month.
Make sure to get it all in writing, though!
~*~Never approach a bull from the front, a horse from the rear or a fool from any direction. - Cowboy saying~*~