One place where this still can make sense is Antarctica. Those "cruises" (more like expeditions) are very expensive on a per-day basis, especially if you don't count two days each way in the Drake Passage. There are businesses (fully legit, operate openly on the main street, nothing remotely shady here) in Ushuaia that sell last-minute vacant cabins on behalf of the cruise lines. If you're not particular about which line you sail with, and can hang around for a day or two waiting for one you like to show up, there are huge savings to be had.