I found loads of stuff.. try doing a search on 'mushrooms nutritional value'
A great site that can tell you the nutrient value of almost any food is:
http://www.nal.usda.gov/fnic/cgi-bin/nut_search.pl
Type in mushrooms or mushroom and hit return. Select the mushroom you want and hit return :-)
But very basically otherwise, I found this :-
mushrooms contain essential nutrients. They are a great source of phosphorus, magnesium, potassium and selenium, nutrients often lacking in our
high-processed-food diets. In addition, mushrooms contain virtually no fat or cholesterol.
They are also naturally low in sodium and a good source of fibre.
Calories:20 Calories, Zero from Fat
% Daily Value
Total Fat: 0g 0 %
Saturated Fat: 0g 0 %
Cholesterol: 0 mg 0%
Sodium: 0 mg 0%
Potassium: 280 mg 8%
Total Carbohydrate: 3g 1%
Dietary Fiber: 0g 0%
Sugars: 0g
Protein: 2g

Vitamins
Vitamin A: 0%
Vitamin C: 2%
Calcium: 0%
Iron: 2%
Riboflavin: 20%
Niacin: 15%
Copper: 15%
Pantothenate: 10%

*Percent Daily Values are based on a 2000 calorie diet. Your daily values may be higher or lower depending on your calorie needs