The most economical way to get to Tegel will of course be by public transportation. In this case you will need a zone A-B ticket for 2,10€ . The price is the same whether you go by bus, S-bahn or U-bahn or a combo of these.
I guess the easiest for you , would be to ride the U-bahn from Kaiserdam to Bahnhof Zoo and change there to either bus X9 or 109 to Tegel.

more info at:

http://www.bvg.de/index.php/en/index.html