Most Americans are paying too much for marijuana. I'm not referring to people who smoke it—using the drug generally costs about as much as using alcohol. Marijuana is unaffordable for the rest of America because billions are wasted on misdirected drug education and distracted law enforcement, and we also fail to tax the large underground economy that supplies cannabis.

On Monday, the New Jersey legislature passed a bill legalizing marijuana for a short list of medical uses. Outgoing Democratic Gov. Jon Corzine says he will...