http://steamcommunity.com/id/ChippieTheAmerican/inventory/ Mine is worth $170 - $180. Most of it is from CS:GO.
When you click on an item in your inventory, you can most likely sell it on the market place. So there is a little green square that reads 'sell' and little numbers underneath it saying how much you can sell that item for. That is how I added it up anyway.
After calculating every single item's worth in my Steam inventory, I can conclude that my Steam Inventory's worth is :- Error 404