> not when the legalized variants are so much more expensive
That's highly dependent on the particular product and regulation. There's not much of an alcohol or tobacco black market in the US as far as I can tell because the regulation seems to consist almost exclusively of a sin tax applied to product sold for human consumption.
Compare this to marijuana in WA, where there's some sort of convoluted and expensive licensing system for cultivating it. Then there's a set of byzantine regulations surrounding the cultivation and distribution itself followed up by a sin tax at point of sale.
If drug legalization were simply that existing pharmacies could sell existing stock to people who didn't have a prescription subject to a tax (less than 50%) the drug cartels would quite literally lose their entire revenue stream overnight. The controlled substances act then becomes a list of taxed behind-the-counter substances instead of a list of illegal substances.
(Bonus: The jobs of academic researches who use such substances immediately becomes _much_ easier. Not for human consumption? No tax issues, just order it from your usual supplier like any other chemical. No more complicated permits and limited availability.)
(Extra bonus: No more meth labs in residential neighborhoods illegally disposing of highly toxic compounds, presenting a serious fire hazard, and otherwise endangering the health and safety of everyone nearby. Industrial chemistry belongs in industrial areas, where it can be subject to OSHA and the EPA.)