Sounds in line with Joe Arpaio's estimate:
"I don't know how to stop the drug traffic, and I've been in it for 38 years," the sheriff, widely touted as the toughest cop in the nation, told Harper's in 2001. "I think if I knew, I'd be the president. I can give you what's been said 50 years ago. ... It's the same thing we're saying today – tough law enforcement, prevention, rehabilitation ...
Nothing's changed. The stuff coming across the border that we catch? Ten percent. Fifty years ago, 10 percent. Today, 10 percent. Nothing's changed ... I don't know how to solve the problem. Don't ask me."
From WND article via http://www.freerepublic.com/focus/f-news/1156176/posts
I’ve never heard anyone in law enforcement say they think they are getting more than ten or twenty percent.