If you have a so-called complimentary cruise certificate from NCL the charges you will pay for includes "guest reservation" (of $175 per person, and everyone has to pay them even though the cruise is so-called 'free') plus "port fees and taxes" which vary depending on ports. This total varies a bit depending on time of cruise, port, type of cabin. These fees total around $300 per person for me for a couple upcoming cruises to Caribbean and Mexico. So that should give u a rough idea.
Fees may be a bit more for Alaska.
This is one reason I feel these "complimentary" NCL cruises are deceptive. Some Harrahs properties describe them as "free" cruises, but $300 upfront isn't free in my book. You must pay these at time of reservation. Getting the insurance is also probably a good idea in case u must cancel.
On top of this you will pay $12.50 per day per person for gratuities. These are billed automatically and paid for with your credit card. If u feel service is bad u can go to the information desk and change this, or if very good, give individual tips to certain people. For me, I just do the automatic tipping, which seems fair enough.
Soda onboard also is not free, and if u drink anything except water, coffee, tea, or milk (except juice at breakfast), you might want to consider buying an unlimited soda package (around $7 a day I think). As a casino guest, drinks in the casino are free (they will give u a special card the first night of sailing if u attend a meeting in the casino, an invitation will come to your room), or ask at the casino. If u are thrifty, you can get cans of soda and cocktails at the casino bar and walk with them to dinner or your room. Otherwise booze isn't cheap. You can buy lower cost bottles of booze on the ship or in duty free shops in some ports, but the ship confiscates them and won't give them to u until u land. Some people sneak booze on board but ship's really try to confiscate any brought on board so they can sell u their own more expensive cocktails. Look for the captain's party or other special events that might have a glass or two of free champagne.
Other major cost is shore excursions. In many cases u can save money by waiting to go onshore on your own and buying from people onshore. Check the Internet for tips on each port and sometimes u can buy good packages in advance online.
On The Epic, and possibly other ships now, u can also book tickets for the shows onboard in advance. You should do so to save time in lines.
For first time cruisers the NCL cruises will be nice. For more sophisticated travelers, NCL isn't the best cruise line.
Food in the main dining room is excellent, and u don't need to spend extra on the specialty restaurants, though some people do and enjoy them. If u eat early, there is no problem with seating but if u eat at prime times u may want to make dining room reservations, despite "freestyle" promises. The buffet is quite good, especially for breakfast, but we found the dining room much better for dinners. You can order as much as u want, so indulge and order two entrees and two deserts if you like, or order something else if the first appetizer or entree isn't so good. Be prepared to gain 5-10 pounds on cruise. Go crazy with the food, it is part of the fun.