There are just too many variables involved in your questions, so you really need to contact your agent - they are your best source of information when considering different options. They'll also know what you mean by 'cheap' and possibly be able to provide other options for you.
I'm always amused by people who first say they want something cheap. What is cheap to one person, might be expensive to another. And cheap often directly correlates to poor quality. To some people, time is more important than money, while others don't care how long the trip takes. Some people may want to fly roundtrip, while others want to cruise roundtrip, and yet others want to fly one way and cruise the other.
A cruise is just totally different than flying in cost, quality, amenities, time, and itinerary, so it's really unfair to even compare the two.
__________________ Travel Agent/Cruise Specialist w/15 yrs exp and 50 Cruises on 12 different cruise lines in many parts of the world! VIETNAM VETERAN OWNED & OPERATED. Certified Accessible Travel Advocate. Specializing in unsurpassed personal service to insure you have a hassle-free vacation!