Although many people consider the gratuities extra and optional, it's not fair to think of them that way. Unfortunately,
DCL does not pay its servers and room attendants enough, and their salary mostly consists of the "optional" tips. Please consider these as a part of your cruise expense.
So, bottom line, your cruise cost is the sum of your fare paid to DCL and the gratuities on board for the servers and room attendants.
You can cruise without spending another penny, although I believe many spend money (some a lot) other items (drinks, wine packages, souvenirs, shore excursions, photos, spa treatment, bingo, art, ship store items, onboard booking, joining DVC, internet, port upgrades, parking/transportation, porter tips, adult dining...have I missed any?). DCL makes it easy to run up a huge onboard account bill.
There are plenty of opportunities to part with your money, yet you can have a truly magical vacation and part will very little extra dough.