When I went on Carribean Princess, the private island Princess Cays was still under repairs.
So, they substituted San Juan and gave us a refund of the port charges for PC which amounted to $20 for 2 PAX.
The timing of San Juan was unfortunate (4pm-midnight) and a lot of people were disappointed. Personally, I found San Juan to be a pleasant surprise and we took a great private tour that cost us (ironically) $20.
$100 seems very generous to me! Of course it's different when a port is 'skipped' vs. being substituted. Maybe Princess figured the PAX would just spend that $100 aboard anyway.