I've never priced a cruise for more than two because I have no reason to. For two people, the price of a Caribbean cruise in April is pretty close. The drinks are very much not free. For my cruise, the "free" drinks were about $436 in "pre-paid gratuities" for all of the drinks that I might order. That's a lot of drinks! I actually don't drink alcohol and, unfortunately, NCL doesn't allow adults to downgrade to the free soda/tea/coffee package. It's alcohol or nothing. If I want a package for other drinks, then I have to pay for it at full package price. And those packages are also very overpriced.
So, ultimately, my room is slightly cheaper, but it's also smaller. There's no self-serve laundry, so I'll end up paying more for that. The soft drinks aren't free. The "free" WiFi is only two hours of time, which isn't much. After that, it's similar in price to Disney's new WiFi system. They charge extra for some food in the main restaurants. The "free" specialty dining did require that I pre-pay the gratuities before receiving service, which is absurd, but it does get me out of the main restaurant for two nights. I guess that's something.
But, again, I'm only evaluating the cost for two people. When you add free kids to NCL, sure, the price difference is significant.