One thing to consider is one's confidence level in planning excursions by oneself. Many passengers will go with the ship's excursions for that reason, even if it's possible to find a similar independent excursion for less.

There is a place on the Princess website that gives a reasonable idea of what excursions are available for a given itinerary. Of course, these are not set in stone until about just a few months before a sailing, and are not available for booking online until about two to three months before the cruise.

Here's how you can find them:
1) Go to Princess Cruises® : Official Cruise Line Website
2) on the front page, there is a "find cruises" link
3) click on that and you'll get a chart. Click on the area that you are planning to go to. You can narrow it down further by ship and date just to get fewer cruises listed. Then click on submit.
4) Click on view itinerary until you find one with the ports you're interested in researching
5) You'll see some tabs that say "pricing and availablity",etc. including one that says "shore excursions" which is the one to click on.
6) You get a listing of various excursions with a description. Closer to the sailing date there should be pricing listed.

By the time that you can actually book any online, the prices will be there. Princess won't be making you book them without letting you know what it costs.