There's an index for the price of just about every commodity, from gold and frozen concentrated orange juice futures (FCOJ) to more media-friendly consumer metrics like the Thanksgiving Dinner Cost index and the Big Mac Index. Especially now that marijuana is legal in Colorado and Washington, it makes sense there's an index for the price of pot, too — and that it is compiled by High Times magazine.

What the March numbers in the index — the Trans-High Market Quotations (THMQ) — show is that marijuana prices are dropping. Not uniformly — as this chart below shows, both high-end and low-end weed have remained pretty steady since Colorado and Washington approved legal marijuana in the 2012 election — but the broader U.S. price index started dropping in December 2013. The March price of $286 an ounce is a record low in High Times' index. High Times doesn't speculate as to why prices are dropping, but the big marijuana event in January was Colorado beginning to allow legal sales of pot for non-medicinal purposes. Colorado is the yellow line below.

If legal marijuana does cause price drops like this, it will be a mixed bag. On the one hand, cheap marijuana will encourage more use of the drug, which will probably have some adverse public health costs. Less expensive weed also means lower tax revenue in Colorado and, soon, Washington. On the other hand, one of the great promises of legal marijuana is that it will undercut illegal sales, especially from cross-border smuggling by Mexican drug cartels. Maybe that's why they're switching to meth and heroin. Peter Weber