eg by checking with the journey planner on http://www.bvg.de/index.php/en/Bvg/Start

Use TXL for the airport and "Potsdamer Platz" for your destination and you will not only find the second to most cheapest way (walking might be cheaper) but will have learnt something new .

Sngly Ticket is 2,10 Euro, Day pass good for all public transport in the city area AB is 6,00 Euro. Enjoy. BTW: Taxi should be around 18 Euro