I've never even considered that, but I think you're really on to something !
I feel like owning a gym would be such a dream of a life, even working at one seems like a really fun job.
Let's do it lol ! We could have a membership package for visitors that is more reasonable than 3100 / day, Maybe 2500 your first day, then 500 the proceeding days you're in town. Maybe also a cheaper joining package with somewhat limited hours for the first month for people who've never joined our gym before, and then after that month, they can choose between the non-limited packages.
Also, instead of banking on the well meaning people who buy memberships but never come, we could have little automated texts sent to phones gently urging them to come in.
Lastly, and most importantly, we'd need lots of beautiful, fit women in very tight spandex bending about the place.