In California, three years into the era of legalization under the Proposition 64 ballot initiative, data indicate that only about one-quarter of weed sold and consumed in the state is legally licensed and that the remaining three-quarters is produced outside legal market channels.
And from a later chapter:
And that is why some people say, and we consider it plausible, that the so-called legalization of weed in some North American markets has illegalized more weed than it legalized.
That is from the new and excellent Can Legal Weed Win?: The Blunt Realities of Cannabis Economics, by Robin Goldstein and Daniel Sumner.