I used to half lease one of my trainer's horses, and it was $250/month (board is $325/month where we board)...I got 3-4 riding days a week and I paid for weekly lessons with him on top of it. But he provided his own tack and paid for anything that may come in health-wise with the horse, I just paid that flat fee per month to half lease with 1 other person.
I've never done a full lease and most people at my barn do half leases, so I'm not really sure what a fair price would be. But I would think that it would be 100% of the board and at LEAST shared health costs (routine stuff like shots, wormers, shoe/trim costs, etc.) if not all of them since a full lease is almost like owning the horse at that point.
But I think I'm with others on here when I say I would not be okay with them leaving the property with my horse - unless perhaps I went with them to a show or something.
"The times when you have seen only one set of footprints in the sand, is when I carried you..."