The way MOST cruise lines calculate the single fare is the following:
Double the commissionable cruise fare
one fuel surcharge
one set of taxes
one non-commissionable cruise fare ( sometimes called port charges)
The rates displayed on a cruise line's retail website usually displays the commissionable cruise fare and noncommissionable cruise fare already combined. This is how most customers get confused.
A single passenger will not pay twice the rate as a passenger in a double due to the taxes, port charges and fuel charge.
For example a 3 day cruise for two might cost $350 per person with everything included
The same cabin will cost $548 for a single passenger with a 200% supplement.
The breakdown would be as follows:
Commissionable Cruise fare 198.00
non-commissionable cruise fare 89.00
fuel surcharge 30.00
The single pas double the cruise fare of 198.00 but only one set of the other charges.
In most cases a single can be converted into a double at any time up until about 5 to 7 days before sailing. The second cruise fare that the first passenger paid will be credited to the second passenger. This second passenger would then only have to pay the other 3 charges (in this case $152) to complete the fare.
Like I said, most cruiseline's have this pricing structure or something fairly similar. Each line may have different rules in regards to this. This is where having an experienced TA is useful. Some TAs may not be aware of these polices so make sure you have them research it prior to booking so that you know exactly where you stand.