To update this thread which I started:

There are several machines outside the bus stops at TXL and they (like all other BVG ticket machines) take 5 cent coins.

I used all my 1 and 2 cent coins to pay the €3 tourist tax which the HI on Rohrdamm charged me, despite being on an award night. As the tax is 5%, this indicates that the stay was worth €60 (or rather €63 with the tax), and that I received 0.6c / 0.42p / US 0.54c value per IHG point

My ticket was checked 5 (!!) times yesterday on the U-bahn and EVERY time 2-3 fare evaders were pulled off the train. These were both locals and tourists. Ticket inspections took place both in the central area and the suburbs.