I don't have figures, but the idea that they have no nutritional value is fairly obviously a myth. The fruiting bodies of fungi are made of the same sort of stuff that plants and animals are made of. They're made of cells, and those cells are made of the same proteins as animal and plant cells. Some of them are particularly adept at picking up metals and trace elements from the soil, and it is for this reason they've been proposed as a means of cleaning up pollution. From a nutritional point of view this could be either a good thing or bad thing, depending on what's in the soil.
These are the figures for "normal" mushrooms:
http://mushroominfo.com/benefits/
Other species will be different, but it is very hard to see how any mushroom could be nutrition-free unless it was totally indigestible.Often grouped with vegetables, mushrooms provide many of the nutritional attributes of produce, as well as attributes more commonly found in meat, beans or grains. Mushrooms are low in calories, fat-free, cholesterol-free, gluten-free, and very low in sodium, yet they provide important nutrients, including selenium, potassium (8%), riboflavin, niacin, vitamin D and more.