There is no established equivalent for the term Cash Cow in Greek, although it's quite popular in other languages. I'd suggest you used Μηχανή Χρημάτων or Επιχείρηση Βέβαιου Κέρδους.
Have a look through the OneLook link provided below; maybe you get some better ideas yourself.

HTH

Alex Seidanis

PS: Searching the web for Cash Cow instances I run onto this funny page. Enjoy: http://www.softlab.ntua.gr/~tkot/vkot/ascii/Cows.txt