It can cost as much as you’re willing to spend.
I would start with a business plan and an outline of everything you want to offer
From there I would look at spaces that will accommodate your list
Once you know what the structure will cost you can them calculate upgrades, construction, and all other additions.
From there, I would get a calculation of equipment , flooring, mirrors, etc along with staff facilities and employee/independent contractor expenses.
Make sure you have a business lawyer, an accountant, liability insurance, and are in compliance with your town and all business codes.
Or, purchase an existing gym
I think it will be helpful if you narrow your question down a little. The variables are so many and all have an impact on the cost. Just to name a few: location, size, projected membership, facilities offered (eg. swimming pool etc.).