In other words, high drug prices give money to gangs. Also, some drugs inherently encourage large gangs - stuff like heroin and cocaine has offshore production, which requires complicated supply chains with large (and therefore violent) gangs. As deplorable as meth cookers are, their small scale and low margins mean they only cause limited damage. Since they work alone, they don't need to shoot disloyal gang members,…
http://www.pbs.org/wgbh/pages/frontline/meth/etc/cron.html
(The gist is that large labs tend to spring up whenever they find a bulk source for either chemical and then go away when that bulk source is eliminated)