Ah. the Disney site tells you the amount per guest per cruise, not per night!! So, it comes out to exactly the 108$ as I said.
One of the great things of cruising is that there are not a lot of hidden costs! If you don't want to spend any money besides the 108$ you could have a perfect vacation without paying anything over what you already paid in your cruise fare. Alcohol, bingo, excursions, Palo, Remy, spa, souvenirs ... all is optional. However, they are really good in making you want to spend additional money for all these services, so beware! It is easy to just get out that Key to the World card and pay and not see how much money you have already spent. I guess that's why they introduced Magic Bands at WDW to get that same effect!
I would definitely recommend using a travel agent that knows DCL for your first cruise as they will be able to help you with a lot of your questions. Also, have a look at the FAQ thread here on the top, lots of great information collected there. Cruising is an easy and relaxed way of traveling, no need to stress out about planning it!! Take a breath and don't be too nervous!!
Hope you will have a fabulous cruise!
And one last tip: If in any way possible, I would recommend to go on a 4-nighter instead of a 3-nighter as it will give you more time on the ship to explore and enjoy and relax!