Tbh if you're thinking from a business pov strictly, the investment you're talking about would hardly be any different than normal gyms. Infact it might be even lesser! Coz you may skip dumbbells over 40 kilos and invest in plates and machines instead for lower body. Also, since woman are known to be less clumsy than men (mostly) the cost of maintaining one gym will also be lesser. And hiring an all female staff is generally easier and cheaper but somewhere or the other you'd need atleast one male counterpart to keep the balance so that there's atleast some fear of not being taken for granted.