I’m not sure if it’s what the author intended, but I understood that to be talking about the actual trip being more expensive. I’ve looked into sea travel before for short (e.g. Melbourne to Tasmania) and longer (e.g. Melbourne to New Zealand), and it consistently ends up a good deal more expensive than air travel because it’s basically a hotel and restaurant for overnight or however many days or weeks or whatever, whereas with air travel that part is minimised, and accommodation and food at both ends can be much cheaper (e.g. free accommodation between one’s own home and friends’ homes).