If you want the freshest cheapest mushrooms around, head to what most locals refer to as the California Mushroom Farms (now Modern Mushrooms) in Ventura, on Olivos Adobe Road. The hours are odd and it's hard to find, but you can get wonderful white, portabella, and more exotic varieties. The quality and prices are far superior to the grocery stores. A whole bag of giant portabella caps is a few bucks. They don't have every type every day, but whatever they have will be good.