Older report from Dell but I just came across it while trying to build a list of strong use-cases for butts: http://www.secureworks.com/cyber-threat-intelligence/threats/cryptowall-ransomware/

"Based on post-mortem data collected by researchers, CryptoWall has been less effective at producing income than CryptoLocker. Both malware families accepted payments via Bitcoin, with 0.27% of CryptoWall victims and 0.21% of CryptoLocker victims paying ransoms in bitcoins. CryptoLocker also accepted MoneyPak, and an additional 1.1% of victims paid ransoms using pre-paid MoneyPak cards. As of this publication, CryptoWall has only collected 37% of the total ransoms collected by CryptoLocker despite infecting nearly 100,000 more victims. CryptoWall's higher average ransom amounts and the technical barriers typical consumers encounter when attempting to obtain bitcoins has likely contributed to this malware family's more modest success. Additionally, it is likely the CryptoWall operators do not have a sophisticated "cash out" and laundering operation like the Gameover Zeus crew and cannot process pre-paid cards in such high volumes."

TL;DR fiat beats butts even in the realm of ransomware. So I'm back to just claiming it's only marginally useful for drugs.