In actuality, if booking this as a group and using the approx. # of staterooms that OP is figuring on, they would only earn (1) free berth (not stateroom) i.e. (1) free berth per 8 staterooms booked @ double occupancy.
OP also, before you can start looking at costs, you need to look at length of cruise, time of year, etc. That can make a huge difference when doing your shopping.