Eye-catching Advertised cruise prices usually start with an inside cabin (no window) and for one person. On top of that, add taxes. Then add port fees. Port fees will vary with the itinerary as these are truly costs the ship must pay the ports visited. These costs are passed on to passengers. Taxes and port fees range from $200-$500 per person usually. If you want a cabin in a better location (upper deck, for example), it will cost more. If you want a window or a balcony, that will cost more. Port fees don’t change based on the cabin selected. Double everything for two people. Even if going solo, you usually pay almost the full price of 2 people.
That all sets the cabin price. From there, things will vary depending on the cruise line. Most will add a daily service charge (or “gratuity”) of about $16 per day per person which goes to the crew who serve you meals, care for your cabin, etc. In the past, you would have given this gratuity to these people at the end of the cruise yourself. Now most lines just bill you for it and handle it for you. Additionally, most cruise lines make you pay for most beverages. You can usually buy an “all you can drink” package at the time of booking the cruise. These are not cheap. While there will always be dining locations on the ship that are included in your cruise fare (the buffet and the main dining room), most cruise lines have added up-charge specialty restaurants — a steak house, for example. There are usually specialty dining packages available when you book the cruise. Other things you pay for are Wi-Fi, t-shirts, etc.
Some cruise lines offer a more truly all-inclusive experience, but these are on the higher end lines and they would not be advertised for $798.
Other costs — airfare to and from the start and end point of the cruise. Potentially hotels the night before the cruise. Or more nights if you wish to spend time in either the start or end port. There is also the cost of doing things in all the ports you visit. Some people do their own thing. Some book tours (thru the cruise line or thru an independent company) Some stay on the ship. If you book a tour thru the cruise line, they are not cheap. Often they will be $150 or more per person for each tour.
So, yes, it is possible to go on a $798 cruise and spend $3500.