How much does it cost to maintain a 15-25 year old ship in very good condition?

In line with the numerous complaints recently about the lack of maintenance in some of HAL's older ships, like those built in the late 90s and older, how much additional maintenance does it cost to keep something built in the late 90s like the Rotterdam or Volendam, or an older ships built in the early 90s like the Maasdam in very good condition, vs. a ship built in the mid 2000s? How much must this add to the fare a passenger pays vs. a newer ship built in 2005? I should note that I was on the Volendam last year and did not see anything like leaky roofs and cracked glass windows or anything like that, and that our outside cabin was in top condition for a 16 year old ship. Also Carnival has a lot of older ships built from 1990 to 1998 or so, do these ships also have the same maintenance issues in their facilities as the older HAL ships?