Price is very market-dependent. For example, I’m tempted to move to @WildGooseChase’s barn, because in my area that would be an outrageously attractive lease price for a nice horse, in a good program, with lessons. My area, $250 would be impossible to find for a half-lease, except for a situation along the lines of, “I keep my horse in my backyard, you can ride him two times a week, but there’s no indoor, only trails nearby.”
From how you’ve described your horse, I think it would be very difficult to fully lease him out, have the leaser pick up all of the costs, and then charge a fee on top of that. There is the free lease option, in which you get no money, but the leaser pays all of the associated fees for horse care, including board (vet care depending on the contract). It’s a bit like the horse transferring ownership to another person for a fixed length of time. Still, that would be a savings on boarding costs. I’ve never known anyone fully leasing out a horse to have the option to “just hop on the horse every now and then,” however. Regardless, for a full lease or free lease, I’d have a clear contract.
Finally, there is a part-lease option. Usually the owner pays all or most of the costs for the horse (like board, supplements, and so forth), the leaser pays the owner a fee to ride anywhere from 2-3 days a week. One or more of those may be a lesson day, and weekly lessons are often required. The owner is unlikely to make money on this, but it can defray some costs keeping the horse, and it’s handy for owners living far away or working a great deal, who can’t get to the barn every day. Sometimes there’s a contract, sometimes it’s done through the barn, and sometimes it’s more casual. Although a contract never hurts.
I’ve also known some people not to lease their horses, but allow the horses to be used in lessons occasionally as a school horse, for a reduction in board. Obviously, you have to trust the trainer to pick suitable riders in that situation.
To be honest, speaking as someone who has always had to lease rather than own a horse, you sound very attached to your horse, and I’m not sure you’re in an emotional place to allow him to be leased out, unless you can find a friend whose riding you trust very much. Of course that’s only natural, and I understand, given many leases can get sour due to the fault of the leaser. As a leaser, I always understood that the owner owns the horse, and has to deal with anything that goes wrong for the rest of the horse’s life or as long as she owns that horse. But it’s impossible and unrealistic to be able to control every ride of a leaser 100%.
The best leasing situations are when the leaser can offer multiple benefits to the owner besides just money IMHO, unless the lease is from a trainer who has multiple horses and that’s part of their business plan. In other words, my best leases were when the owner needed someone to keep an older horse in work, or had a pony she’d outgrown but still wanted to keep. The money for a half-lease often isn’t enough to defray the anxiety and resentment of an owner who has one beloved horse she’d rather have all to herself, but is part-leasing out because she can’t afford him.
I agree that you need to sit down and determine just how much money you need and want to save, and work back from there.