According to a recent report,

18% of the $263 billion spent on retail prescription drugs in the United States in 2012 was paid out of pocket.

Source: "Strategies Used by Adults to Reduce Their Prescription Drug Costs: United States, 2013," Centers for Disease Control and Prevention, NCHS Data Brief Number 184, January 2015, http://www.cdc.gov/nchs/data/databriefs/db184.htm