These are almost never included in the BOGO discount. Even if a guest's fare is listed as "$0," they must still pay mandatory government fees and port expenses, which can range from $150 to $500+ per person.
Prices are calculated based on two people sharing a cabin. Solo travelers generally do not benefit from BOGO deals and often pay a "single supplement" fee that can be up to 200% of the per-person rate . Common BOGO Variations by Cruise Line
In the cruise industry, BOGO is almost always a "Buy One, Get the Second for X% Off" promotion.