Fanatic, you're quite right. The value of the free cruise berth(s) usually equals the value of the type of cabin that has most of the group's bookings. If the group sails in 5 inside rooms and 4 balcony rooms, the free cruise will equal the cost of the inside berth, not including tax.
Over the years, I've seen the free berth be a minimum category no matter what type of rooms the group has, and I've seen the free berth be the minimum category that the group is holding, no matter how many rooms are booked in other categories. I'm glad most of those policies have gone by wayside.