If it's a on property lease I'd charge them board and vet care.
If it's part boarding then $200 a month
My current barn bases it on partial/half/full lease for partial it's 275$ a month because we pay for farrier and partial vet, for a full lease we pay board 300$ a month. At my old barn they did 100$ a ride, so if I went out 3x a week that's 300$ a month. Not including showing costs/lessons. But it all depends on what they can afford and what your looking to do.
Depends on a bunch of things. If he's staying at your barn and the lessee will be the one riding him more often, they normally cover board, shoes, and all his other living expenses. If you both will be riding, you split it depending on that ratio. If he's going off-site, they would already be paying their barn's expenses, so you would normally be charging a lease fee. Also, your local economy plays a part. A $2,500/month lease makes sense in some places for a competition horse, but obviously that doesn't sit well with a lot of people. My biggest piece of advice though is to write a contract up.
Most horses the price depends on skill level if its a trouble ride or not and how many times the leaser will go out to ride your horse normally its around 200$