If the ship cannot substitute a different port for the one missed, they give you an onboard credit for the actual port charges for that port. That is why the amount can vary. It is not meant to pacify upset passengers, but merely a refund of something that they charged you for to begin with. In reality, though, I have never missed a port that was not for a very good reason. Usually a storm, hurricane or other. Or some kind of political or dangerous activity in the port. It can also be some kind of damage to the dock where the ship would, well...dock.
The cruise lines have, not just the right, but the obligation to put your safety ahead of anything else, including your wishes. And, as already stated, using a cruise just as transportation is foolish. If you have to be somewhere or are desperate to see a particular place, fly there. Would I like some money for every port I have ever missed? Oh yeah. Do I feel entitiled to it in any way? NOPE.
One last thought, it is not a simple thing to find a substitute port. Each one has a limited number of spots and there can be a lot of ships affected by something big, like a hurricane. Plus, there are issues of differing port charges, sometimes tendering problems, and the different laws in each port regarding immigration, etc. The only times I have seen substitute ports were one Mexican port for another, or one Bahamian port for another. The last, but not least factor is getting to a different port, which can depend on fuel usage, length of time in transit and probably a bunch of stuff I have no clue about.
So, follow the advice given...relax, know that you are in the hands of a captain who insists on keeping you safe, have a drink up on deck, an extra slice of pizza, whatever makes you happy.