The Internet makes it easy to compare prices for a lot of things -- you can shop around for the best deal on shoes, or research what salary you should be asking for in a job interview. Now, it's also helping people make sure they're not getting ripped off on the black market.

PriceofWeed.com, a website that asks people to anonymously submit the cost of marijuana they purchase in their area, has collected a lot of data on the street price of both legal and illegal marijuana around the U.S. over the last few years. We took that data and created a map of state prices and a graph of city prices below.

According to the site, the average price for an ounce of marijuana in the U.S. is now $286.35. In the green states, the price is lower than this average; in the darkest green states, it is more than 10 percent lower. In the yellow, orange and red states, the price is higher.

As the map shows, marijuana is now significantly cheaper throughout most of the West, as well as Florida and Mississippi. It remains the priciest in Virginia, Iowa, North Dakota, South Dakota, Wyoming, New Hampshire and Vermont.


Source: Priceofweed.com

These graphs of prices at the city level show a similar trend, with marijuana prices significantly cheaper on the West coast than the East coast:


The reasons for price trends aren't crystal clear, since marijuana is still illegal in many places (including federally), and there isn't a lot of data on growing and distribution. However, the price trends appear  related to the supply of marijuana in the U.S., which is in turn partly linked to state-by-state legalization and decriminalization of the drug.

Currently, Washington, Oregon, Colorado, Alaska and Washington D.C. have all legalized recreational marijuana, and medical marijuana is legal in roughly two dozen more states. Although states that have legalized marijuana often add hefty taxes, the increase in supply that comes with legalization tends to drive down the price in those areas anyway. Whether it's legal weed or street weed, the presence of increased supply of both medical and recreational marijuana around the U.S. is pushing the prices down.

Floating Sheep, a blog that created a map of marijuana prices using PriceofWeed.com data in 2011, also attributed the price differences to variations in supply. The blog noted the lower prices near areas with a lot of marijuana production, including Mendocino, Trinity and Humboldt County in California -- a huge marijuana-growing area known as the "Emerald Triangle" -- as well as Kentucky and Tennessee. (In 2012, a profile of California marijuana growers by the Los Angeles Times discussed how indoor cultivators around the state and industrial-sized operations in the North Coast counties were giving the Emerald Triangle a run for its money.)

One mystery is D.C., where prices remain high despite our (sort-of) legalization in February. The high price could have to do with a lack of officially sanctioned sales. The District's rule change made marijuana legal, but prohibited pot shops, open-air smoking or exchanging of marijuana for money, though barter is okay.

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