Just spent a week in the Witsunday's and most of the +100ft plus yachts anchored as they were to big for the marinas. However Puddinlegs is right, for a 100ft plus yacht mooring costs a minor compared to crew costs, r&m, fuel, etc, etc. Assume a 20% annual running costs for a $1m yacht and you are talking at least $200,000 per year.
The cheapest +100ft yacht we saw was worth $20m and the majority was worth a lot more. Here is a 136ft yacht that was anchored behind us, I don't think the owner would be too concerned with mooring costs.