Your post here is slightly ambiguous, so I thought I'd add to it a bit in case you are falling into a common trap.
>> Divided at the per-visit or per-customer level, you can see then that every single visitor incurs some cost.
Your statement above is correct if you pay the insurance amount based on a per visit model. In other words if you report the number of visits to the insurance company every month, and your premium changes monthly. I've no idea if this is what's happening or not. If it is, then your post is fine, but if the insurance is a fixed amount then you've fallen into a common trap. Which is this;
You cannot average a fixed cost across variable use, when calculating marginal cost.
For example let's say I have a factory making various steel goods. Mostly I make fences to order. In my slack time (which all factories should have) I get otherwise-idle staff to make shopping trolleys. I supply those regularly, and so it never hurts to have stock of them.
Now, here's the question - should I add labour to the cost of those trolleys [1] or not? As labour is a fixed cost I'm paying the employees regardless. Whether they make trolleys or sit around drinking tea it costs me the same.
A costomers walks in, and wants to buy the trolleys at a discount, which covers all the materials, and half the nominal labour rate - should I sell them? The right answer is yes (better to recoup half the labour cost than none of it) the wrong answer is to say no, the labour cost was x so selling at half x is losing money. [2]
So if your insurance is say 1000,thats a fixed cost and is not part of any specific gym visit. Adding "free" visits has no impact on the insurance. The marginal cost is very much only the actual costs of that visit - which are likely close to zero (some water in the bathroom maybe?)
[1] I am assuming staff are paid hourly, not per job, are are not "sent home" during idle time.
[2] I get this is simplistic, and other factors can come into play. Like if I have limited supply and am prepared to wait for a higher offer. But presuming I have the ability to create (in idle time) more than demand, the question holds.