Cheap and good deal aren't necessarily the same thing on a cruise and knowing your cruise wants greatly helps finding a good deal.

If you are looking for a low cost per night on a mass market line (Carnival, Princess, Royal Caribbean, Norwegian, etc), also check the extra expected costs beyond base fare + taxes like gratuities, wifi, start/end port (some are only cheap for drivers), and drinks if you like soda or alcohol. If you're into food, look up the main dining room menus vs specialty dining menus + pictures/reviews & add on specialty dining fees. Cruise line food quality and variety do differ. The base fare might be less but if you'd really like specialty dining, a drink package, and wifi then you should compare including those costs as they can vary from line to line or even sailing to sailing.

A good cruise oriented travel agent can be quite handy, even for those who like doing their own trip research. I do my own travel planning but still generally use a cruise travel agent. They've saved me a lot of time when needing to ask questions of the cruise line and when there are changes in addition to the value add perks gretchendz mentioned.