Cruise lines generally charge a "single supplement" to passengers traveling solo. From the cruise line's perspective, what they are selling is cabin space, and if there are two or more people in a cabin, they split the cost of that space. People traveling alone pay for all the space by themselves.
Most cruise lines start with the per-person rate based on double occupancy and add 50-100% to the price for a single. That means that a cabin that sells for $1,000 per person based on double occupancy would be sold to a single for $1,500 to $2,000.
At Vacations To Go, we're sympathetic to our single customers' desires for a great price. We've scoured more than 25,000 departures to identify and secure the lowest single supplements for our customers, down to and including great cabins that can be reserved by our single customers with no supplement at all.