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Actual trans-atlantic liners were built for much heavier seas and conditions in general compared to modern coastal cruise ships.
The QM2 may have been the last one that will ever be built.
Current price on Cunard's website shows a 14 day cruise from UK to South Africa starting at $2179 per person. That comes out to $155/day which is the price of a 3 star hotel in the USA most of the year. So realistically it isnt that bad for what youre buying. Though they probably gouge you on lots of little things like drinks or extra ship privileges.
Ironically i've heard of container ships sometimes offering passenger accomodation but it ends up costing almost as much as the QM2 is charging, for far less amentities.