As Booth reported:

A 2010 Rand Corp. study estimated that legal marijuana use in California, a state that consumes about one-seventh of all the pot smoked in the United States, would cost the cartels 2 to 4 percent of their revenue. So losing consumers in states such as Washington and Colorado that have a smaller population might not affect the cartel bottom line by much.

It’s hard to determine exactly how U.S. marijuana legalization would hurt the cartels, in part because we don’t have perfect numbers on how drug traffickers profit from marijuana use in each of the 50 states.
The Rand authors looked at the hypothetical case of marijuana legalization in California, which was a proposition in the state at the time that didn’t pass. When they tried to determine the impact that access to legalized marijuana in California would have on the cartels’ market share, they found that marijuana exported by Mexican “DTOs” (or drug-trafficking organizations) dominates the New Mexico and Texas markets but doesn’t saturate the California and more northern (Oregon and Washington) markets.