How does a tire wear out while in flight?
At the club where I fly there's a fixed cost per flight. This obviously also covers wear and tear while on the ground (insofar a grass take-off or landing wears out a tire in the first place - most tires need to be replaced due to old age, not due to wear). But you don't get charged for airtime as airtime doesn't cost the club a whole lot.
The only thing directly related to airtime is the 50-hour check. And that one is done by volunteers, for free.