Mana Mushrooms

Another program from USDA.

Information on everything from price to sales to production broken out along various axes in spreadsheets.

USDA Data

Plenty of market information here, including pricing and other useful information about sales and advertising on a broad scale, separated by region:
USDA AMS

The Economic Research Service of The US Department of Agriculture provides some great research through this site:

USDA ERS

The information is provided by state, for the US as a whole, and by country breakdown for imports and exports

This document, also compiled by the NASS, takes 2001, 2002, and 2003 data into account, showing a slight increase in sales and value of mushrooms. It includes state and regional information, including sales volume and per pound pricing, further broken down to brown, agaricus, and specialty mushroom categories.

NASS_USDA_2004_Market_Report