Just to give what I can: I have met very, very, very few people who open a gym like you are talking about who do it for money. It's almost always a passion project, and even the ones who do end up making a little money (which almost always takes years of operating at a loss, while also working ridiculous hours), usually have to supplement their income for their typical bills (home/food/car/etc.)
The small profits are typically used to maintain equipment that's been broken over the years, or hire what little help they can (a lot of these gyms have people who train there as regulars who help out and keep the gym open while the owners work, I did that for my friend when he had his gym. And know of two others that do the same in Houston).
Typically, the people that open a gym like this for money end up out of business.
There was a guy on the other BB forum who opened a gym in Ohio a few years back. It looks like it is a really nice place, but his initial investment was limited to the space. He ran a successful used gym equipment business for years that allowed him to make quite a bit of money flipping equipment and helped him find a lot of stuff that he wanted to keep first. Then when he decided to open the gym, he didn't have to purchase much else besides the space and flooring. I also understand that he continued the equipment flipping business and runs it out of a different part of that same building.