Economics of scale. That's why ships exist that can carry more passengers than the populations of small towns, plus crew. 5,000 passengers and 1500 or so crew is just crazy. It's a far cry from the "Love Boat" era of the mid 70's to mid 80's. Pacific Princess (Originally Sea Venture, launched in 1971) carried just 750 passengers and 350 crew. In 2002 she was replaced by another Pacific Princess, formerly the R Three (built 1999), an operated by Carnival at a capacity of 826 passengers and 373 crew. The configuration of the ship is much more towards rooms with balconies. The second Pacific Princess was sold by Carnival in 2022.
The current biggest ship in the Carnival fleet are of the Excellence class, with a double occupancy passenger capacity of 5,282. Maximum capacity is 6,500 - on the Mardi Gras owned by Carnival. Crew is 2,000.
But that comes up short behind Royal Carribean's Oasis Class Wonder of the Seas with 6,988 max passenger capacity and 2,369 crew. But when their Icon class flagship Icon of the Seas enters service next year it will pack in up to 7,600 passengers and 2,350 crew.
So by packing in so many paying passengers, you, and about 7,000 people you don't know, can go on a fairly cheap cruise.
I hear its far better to splash out on an all-inclusive package because the el-cheapo rate for a stateroom will nickel and dime you into the poorhouse with all the amenities, food etc. costing extra.
Another factor in low-ish cruise prices is the cruise lines need money. The pandemic had them shut down for 3-ish years with no income. They sold off or sold for scrap most of their ships built in the 1980's and 1990's while not stopping investment in new, higher capacity ships. That money influx kept the bigger cruise companies afloat, along with the income from the pre-pandemic time.
Smaller companies bought still serviceable cast-offs (like the second Pacific Princess) and spent time and money during the pandemic shutdown getting them fixed up and they've spent the past couple of years working like mad to pay for those ships. The ship breaking yards did a huge amount of business scrapping hundreds of old, and some not so old, cruise ships. Some built as recently as the mid-1990's went to the breakers recently, along with some built as far back as the early 1970's which were still in service until their owning companies couldn't weather the shutdown.
But wait, the Icon class ships will soon be looking like pipsqueaks. The Global Dream and Global Dream II are built to carry up to 9,000 passengers! Unfortunately the company having them built to serve the Chinese market went bankrupt. Global Dream was finished and Global Dream II was not finished. Both ships must be moved from where they are by the end of 2023 because the shipyard has been sold to a company with a military contract. They were destined to make their maiden voyages to a scrapyard but Disney bought Global Dream II "for a favorable price". I assume because being unfinished it was available for less and because it will be easier to configure for the western market that to redo the completed ship. I don't know if anyone has bought Global Dream. Disney expects to have their new ship in service by 2025.
How much? Reportedly over 1.5 billion UK Pounds had been spent on Global Dream II, and 2 billion on Global Dream.
Makes me want to sing "Do you want to buy a cruise ship?" to the tune of "Do you want to build a snow man?"