I have one of these coming up next month, for a group of 14. It's a private trip so no-one else can join. I regularly get asked about these trips and my answer almost invariably is that we can't do it.
To absorb the cost of fuel and charge what people will pay needs a minimum of 8, probably 10 people, and we never have anything even approaching these numbers at any one time. The customer base here is just too small, and currently contracting.