We spend a lot of money on a personal trainer; and a coach specific to my poison (cycling). The personal training and coaching are the greatest expense and I will allow my lights to get turned off before I let that lapse - it's that important to me.
There has been a significant investment in a rowing machine, indoor smart trainers for cycling, and quite a bit of free weights. My wife runs, and she spends a healthy amount on running shoes.
There are monthly subscriptions for things like Trainerroad, Zwift, and "upscale" yoga. The yearly cost of all three might be $500.
I invest in a DEXA scan at least once a year for about $50. Workout clothes wear out and need refreshing, but we go cheap on that and the expense is minimal.
There are periodic expenses - a new Fitbit, or a heart rate strap needs to be replaced - but they are not frequent or costly.
There is a large upfront cost to all of this - I now mention the 2 pretty expensive road bikes we use, but they are years old and have more than paid for themselves by helping us attain better heart health - but it's not prohibitive.
I will also note that we have consciously chosen this route vs. maxing out 401(k) or similar. We are not wealthy (in the financial sense) nor are we sitting on a nest egg or in line to inherit a large sum of money.
Finally, we're not monomaniacal about fitness. We like good food, going to movies, socializing, and so on. We could do 3 nights in NYC for about $2500 - or we can skip that and use the money for new equipment or a trial run at, say, a new gym. We choose the later 9 times out of ten.