We taught a workshop at a folk festival a few years ago on cittern/Irish bouzouki/octave mandolin. We had nine instruments in the room and seven different tunings. My instrument has five courses, tuned D-G-D-G-D.

Some other common tunings:
GDAE
ADAD
GDAE
AEAE
GDGD

I think a new Sobell cittern is in the $7000-$9000 range.