My T has a policy. Cancel within 48 hours and you pay the full fee. Cancel between 48 hours and a week before and you pay 50%. Cancel a week or more before and you pay nothing, but you can only do that (cancel as opposed to rescheduling) for up to two sessions a year.

I think it's fair enough as he's in private practice. I've never cancelled. And my T says he's been off sick three times in 20 years, so it's not like he often cancels on clients.