Agreed.
By the time you have paid gym fees and insurance you will be on next to nothing if your target market is average Joe. Very congested market and IMO you need a big edge these days to stand out: superb shape, private gym, pro status, medical/scientific quals or as much as it pains me, social media status.
Any of the above, you stand a chance of earning good money.
Being a 'Rent a trainer' in planet fitness working with Joe public isn't going to set you bank account on fire.