Private gym ownership is tough, although with many smaller gyms like Curves, and variuous private trainer centers opening up, you never know.
You need to know your gym inside and out, what works, what doesnt, making sure that the money coming in matches what is going out, with no skimming, etc.
Memberships are one things, but there are a lot of other aspects. You can have a small store in there, selling drinks, bars, supplements, although you need someone who understand how to run it. Sometimes these stores can bring in a ton of money, and that contributes to your bottom line, or you rent the space out to someone who is willing to take care of it.
You need to understand the base of your clientle, whethere it is hardcore members, baby boomers, young people, women, non-bodybuilders, and so on. Make sure people are comfortable. But with the gyms I visit, each has its own unique style, whether MMA, mainstream oriented, hardcore oriented, or a mixture of both. Some have day care in them, some of valet parking, and so on.
But it is an investment, and you need to understand that you will be totally into it. You can't just expect it to thrive. Any gym must be built with hard work, and be willing to do whatever, even loading your own boxes, and so on.
And I would assume location, location, location is so important too. But hey, I am not an expert on this.