I'm not a fan of ship tours, as I don't like getting on a giant bus, and I find them overpriced. They do have the advantage that if the ship doesn't make the port call they are refunded, and they will make every effort to hold the ship for you if your tour is late (but that is by no means certain). As for cost, well, they are pretty outrageous.
I just booked a Golden Circle mini-bus tour in Reykjavik for $97 which is non-refundable within 24 hours of the tour. The same tour from Princess, on a giant bus, is $295. Perhaps the most ridiculous price difference I've seen is for a Fly Over Iceland attraction that is sort of like the Soaring ride at EPCOT, maybe a little better. Princess is charging $155. I paid $36. Like the Troll tour, I have a 24-hour cancellation window, so there is some risk, but I think there is a better than 25% chance we will make the port call.
Sometimes Princess does offer simple ride-there-and-get-off tours that are pretty good value, simply because they are easy to book, but I'm just not going to pay three or four times as much for an experience that isn't nearly as good as what I can get on my own. I know this is nothing new, but the Fly Over Iceland price difference just kind of knocked me down.