it depends on what the pony is capable of doing and if youre going to make the leasee buy their own tack for the pony or if you're gonna supply it. same w, are you going to pay board/vet bills or are they?
before i had my own horses i leased a gelding. it depends on the horse. the gelding i leased was an extraordinary jumper, very talented jumper. i rode him every day and was allowed to take him to the trainer and shows i pleased. it was like my own horse. i payed 300 per month. fair deal