I was reading an article about how the price of marijuana responds to law enforcement. I learned of a neat website called PriceOfWeed.com where people can anonymously post how much of which strand of marijuana they purchased (including quality level) at what price. I don't know how much I trust the data, but it's notoriously difficult to get high quality data on black market transactions.
The basic finding is that :::surprise::: weed is more expensive when enforcement is heavier! One interesting result that I had never thought about (maybe because I'm never in the market for illegal drugs...) is that latitude matters. It appears that pot is more expensive the farther north you go, in richer countries, when it's higher quality, or when you purchase it in lighter weights.
The article is:
Thies, Clifford. 2012. "The Relationship Between Enforcement and the Price of Marijuana." Journal of Private Enterprise. 28(1):79-90.