Upvote:8
In this day and age, β¬120 is not that pricey and it will be difficult to find something dramatically less expensive without losing so much time as to make it impractical for short visits (otherwise taking the bus would be worth considering). Your best bet is therefore getting a rail card (Bahncard with Railplus?) or Europa-Spezial fares to reduce the costs of the train ticket somewhat.
Also, book the next 2-3 trips in advance, don't wait to return to buy the next one. The earlier you book, the easier it will be to find special fares.