The queen mary 2 is expensive today because it is nothing but a luxury tourist trip. Back in 1950 when there was serious ocean liner travel, an ocean liner ticket was much less than a transatlantic flight. Trains were also substantially cheaper. It is only absurd today to travel long range by ship or by train because people overwhelmingly paid a premium for faster travel to the point that the market for the other methods collapsed.
I suspect that it's mostly that air travel has gotten a lot cheaper relative to other modes of travel. It's true that you can't really travel "in the back of the bus" in an ocean liner any longer but I doubt if roughly equivalent first class tickets and dining are all that more expensive today. (Although I haven't done the calculations.)
Even today, when there is essentially no utilitarian travel by ocean liner, a transatlantic crossing in a balcony room on the QM2 is $800. An economy flight from new york to paris on Air France is $849. First class is $9,262.
That sounds more like what I'd pay for a round-trip economy ticket to Europe and there are probably going to be other costs for lodging etc. associated with the ship. But, fair enough, like (sometimes) long distance train, air isn't necessarily hugely cheaper.
(And, yes, business or first class that you actually pay directly out of pocket for is priced like a luxury good. Of course, you can pay a lot more for the cabin too.)
