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Prices in the cruise business can be, appropriately enough, extremely fluid. The Royal Caribbean`s Sun Viking and the Radisson Diamond were chosen, somewhat arbitrarily, for this report to illustrate some of the differences between a moderately priced, ”average” cruise and one in the luxury category.

When we booked our trip, Radisson Diamond officials were quoting a price of $2,400, or $600 per night per person, double occupancy, for a four-night Caribbean cruise. Add to that port charges of $60. Add another $100 if using the company`s air program from Chicago or other major cities in the middle segment of the U.S.

The air program includes transfers and baggage handling between airport and ship. Those not using the air program may deduct $350 from the total cruise fare (which lowered our per-person, cruise-only cost to $527.50 a day). The two Owner`s Suites are priced at 150 percent above the quoted fare.

However, the Radisson Diamond since has begun offering a two-for-one special rate on certain dates through January on Caribbean cruises, so during that time at least, this luxury ship`s rates will be only slightly higher than the Sun Viking`s. And Radisson executives say they are reexamining their permanent price structure.

A larger ship, such as the Sun Viking, has many price categories. The $1,795 cabin described in the accompanying article was an outside stateroom with two twin beds, one deck above the bottom public deck. Add to that an additional $80 for port charges.

Our cruise was booked during the ”value season” in November, and the price included roundtrip airfare from Chicago and transfers. (Those not using the air program may deduct $250, which brought our per-person, per-day cruise- only price to $232.14.)

Prices (double occupancy but not including port charges) for a seven-day Southern Caribbean cruise on the Sun Viking in 1993 will range from $1,265 a person in an inside cabin on the bottom deck during Economy Season (departures on Dec. 4 and 11, 1993) to $2,965 for the Owner`s Suite in Peak Season

(departures from mid-January through early April). The cabin described in the accompanying article will range from $1,815 to $2,015 in 1993, depending on season. All prices include air fare from Chicago.

Except in its lowest-priced cabin categories, Royal Caribbean offers advance-purchase discounts for passengers booking at least six months ahead. Those discounts trim 5 percent off the price in the peak season, 15 percent in the Value Season (early January and mid-April) and 20 percent in the Economy Season.

Fares may change without notice, and travelers willing to book at the last minute or well ahead of time may find bargains even among the luxury ships. Consult a travel agent.