If you would like to occupy a single room cabin on the trip, then you will have to pay a surcharge.
If you would like to share a cabin, but do not have a travel companion, then most operators offer the possibility to share the cabin with another same-sex traveler. In this case, there is no extra charge. Most of the time, these encounters with like-minded people are very enjoyable and, with a little luck, you may even have the cabin to yourself if no other guest with the same desires could be found.
Some ships also offer explicit single cabins, which are smaller and then do not stand out so much in terms of price.