From Reason's Hit & Run blog:
Using consumer-submitted data collected by PriceofWeed.com, University of Kentucky geographer Matthew Zook, Oxford Internet Institute research fellow Mark Graham, and University of Arizona graduate student Monica Stephens produced this map, which shows where in the United States pot is relatively cheap and where it is relatively expensive. The map, which is reproduced in the September issue of Wired, is based on more than 16,000 anonymous purchase reports, with outliers removed. "One of the things that jumps out clearly," Zook et al. write, "is the low prices associated with the marijuana production sites [of] Mendocino, Trinity and Humboldt [counties] in California as well as Kentucky and Tennessee." Nationwide, average prices were $377 per ounce for high-quality marijuana, $245 for medium, and $138 for low.
For more analysis of the data, see Zook et al.'s working paper. Brian Doherty noted the Price of Weed site last September.