I would suggest looking at multi city trips for the itinerary. Multi city can string together those cities and be more affordable than individual one way fares. And if it helps with the pricing, you could add the Prague-Berlin segment to make it a complete round trip and then just "miss" that flight.
Although I would not suggest doing the throwaway ticketing on a regular basis, If you happen to "miss" a segment of one ticket you bought is it really worth it to the airline to chase you down for a couple hundred dollars? No, it isn't. How are they going to know if you really missed the flight due to unexpected delays, just didn't want to fly that day (maybe you were sick), or purposely threw away that segment? With the outrageous change fees most airlines charge, why would you even attempt to change a cheep ticket and pay more for the change than a new ticket might cost?
The worst that would happen is they would kick you out of their frequent flyer program and confiscate your accumulated miles if "missing" flights becomes a regular thing for you.
Just thinking about this reminded me of a recent situation where I missed a flight on United. I used their app to change my return segment because I wanted to stay a little longer. Unfortunately I chose a flight leaving a day sooner instead of the later flight I thought I was choosing. United's response when I asked was "Buy another ticket." Far from chasing me down to fine me for throwing a way the return portion.