An upcoming WINGS event has instructors volunteering their time to
participants for the flying portion for free. However, the notice says
that satisfied customers will not be discouraged from tipping their
instructors.

What is considered an appropriate tip for this situation (assuming I'm
a satisfied customer when all is said and done)?

Typical instructor rates at the local schools are $40-$50/hour. They
select some of the top instructors in the area for the seminar, though.

Anyone have an opinion on this? Any instructors volunteer their time
to a WINGS program want to comment on what they would expect?