As Tom suggested, daily or frequent trips would be cheapest with a monthly or annual pass. A few trips each week, then you'd have to check the price difference between the pass and "mehrfahrtenkarten", multi-trip passes. My kids only travel to or from school without me on rare occasions, but they each have a card in their wallet to clip when traveling without me. You can buy them in bulk, either at a ticket office or online.
Curiously, the
online ticket shop appears to not be available in English.