Oregon weed isn't just the cheapest in the country. Its wholesale prices for outdoor-grown cannabis flower are now the lowest the United States has ever seen.

Those wholesale prices hit an all-time low last week at an average of $398 per pound, says Cannabis Benchmarks, which monitors industry prices nationwide. The group says the Oregon price is "the lowest ever observed for that grow type in any market."

That's a result of a massive oversupply of recreational marijuana since last fall's harvest—a glut that is putting growers out of business because their product has so little value.

Leading cannabis economist Beau Whitney says he is not surprised prices keep falling in Oregon. "There are still growers entering into the market, and because they're entering into the market, there's going to continue to be a healthy supply at least into 2019," he says. "We can expect to see continued price declines for at least eight to 12 months."