Specifically, Symantec warns that:

 

An investigation into one of the smaller players in this scam identified 68,000 compromised computers in just one month, which could have resulted in victims being defrauded of up to $400,000. ... Given the number of different gangs operating ransomware scams, a conservative estimate is that over $5 million a year is being extorted from victims. The real number is, however, likely much higher.

One of the reasons the scam is so successful is that computer users tend to pick up this malware when they visit a porn site. The computer user is then shown an image purportedly from a government agency, like the FBI, claiming the computer has been locked because the computer was used to view pornography. Sometimes the ransomware message claims child pornography was found on the computer. And then it typically says the user must pay a fine within 72 hours or face arrest.