I agree that they're grossly overpriced, and while IN THEORY they keep "the wrong people" from coming into the country, the reality is that they just keep honest people jumping through hoops. They're also outdated because they don't fit into a wallet nicely.
I also can't buy into the idea that they're inexpensive because they're good for 10 years. How many times will you use that passport in the next 10 years? If -- like my husband -- you're going to travel for business and will use it several times a year, it's money well spent. Or, if you're a family who's going to travel on non-cruise vacations, you may NEED a passport for some destinations. On the other hand, if -- like my kids -- you're going to go on 1-2 cruises before your passport expires, then it's an expensive proposition.
An analogy: Which is more expensive? The $15 dress that you wore once, or the $100 jeans that you wear twice a week for two years 'til they fall apart? Sure, the dress didn't cost much . . . but in the long run, given that it got so little use, it was a waste of money. A passport that sits in the safe for 9 years and 50 weeks of its 10 years is the same -- it's pretty expensive when you consider its "cost per use".
Back to the OP:
If you're looking at a typical Caribbean cruise that leaves and returns to Florida (or another American port), you can cruise with just a driver's license and a certified birth certificate (be sure it's a certified copy). Those documents will get you on and off the cruise ship without any problem. They don't take any more time, and they aren't any more inconvenient.
Of course, the doom and gloom squad will warn you that 1) if you have an emergency and need to fly home halfway through your cruise, you can't do that from foreign soil without a passport. Of course, you couldn't fly home 'til you reached an island anyway, and that could be 2-3 days . . . and some of those islands are US soil. 2) if you were stupid enough to miss the ship at one of your port stops, it'd be both trouble and money to be able to get back to the US. Yes, these things could happen, but they are highly unlikely. Everything you do -- even driving to work each day -- entails some risk. You have to decide upon your personal comfort level.
Another option: The passport card. This costs much less -- I want to say $20, but I'm not sure I'm right on that. It's a wallet-sized card, and it allows ground travel within North America; thus, you can use it really just for Mexico and Canada. However, it is more expensive than the birth certificate option, and it doesn't allow you to fly, so for this application it has no advantage over the birth certificate.