There's a further Turkish operator offering a prepaid product in Germany as a MVNO which includes cheap rates while roaming in Turkey:
TÜRK TELEKOM Mobile - Tarif
Incoming calls (to your German number) are free
local calls and calls to Germany € 0.15/min
international calls to any other destination € 1.29/min - 1.99/min
data costs € 0.30/MB
free Internet access on all TTNET WiFi hotspots in Turkey for 600 minutes/month