The prices for developing have been different on every cruise line I have been on so I can't say what the price would be for you to develop your film.
I have never seen a deal where the more rolls you develop the cheaper the deal you get. I have seen deals in the daily newsletters in the past for deals on developing or buying film. Make sure you keep an eye out on that newsletter.
Usually you drop off your film one day and it is ready by around 6 the next day. At least on the ships I have been on. This eliminates having film developed quickly on the last day.
There isn't a darkroom available to process your film yourself.
Processing is much more expensive than the local drugstore. The industry counts on you being in a spending mood. Now there is the incentive not to take exposed but unprocessed film through airport security.
No break for lots of rolls at once.
There is no last day. There is a last wake up, breakfast and off the ship as the sun is coming up. The photo shop will stop taking film the day before when they can no longer get it processed and ready for pick up the next (last) morning.
it is more expensive to get your film developed on the ship, but it is great especially if you are traveling in a group, you can trade photos get negatives and things like that, its just fun to relive your adventures right away instead of waiting until you get home