The general rule for an annual full lease is 1/3 the horse's purchase price for the year. So (unless you want them to sign a yearly contract) you'd figure out what that is per month and for a half lease you'd cut that monthly fee in half. The way we do it at our barn, 1 person has 3 days with the horse, the other has 3 days, and the horse gets a designated day off. The leasor would pay for half the board, half the farrier, and half the routine vet bills OR any vet bills that were caused by their part (injured while they were riding, etc as opposed to injured while running and playing in turn out. In that case the owner pays for that). To make it easier for us we actually just prorate the farrier/vet per month since our horses are on a 6wk schedule for farrier and they don't get wormed/shots every month!
PS you have a VERY cute horse!