According to IMS Health, the 2013 U.S. market for opioid analgesics was $8.3 billion. Of this, extended release opioids accounted for approximately $4.3 billion. The market leader, controlled release oxycodone (primarily OxyContin®) had annual sales of $2.5 billion in 2013 (IMS Health).
So ...$20 billion "worldwide" seems plausible to me.