Steam has added an option that allows you to see how much you’ve spent on the platform. You may regret looking at it.
The “External Funds Used” tool – quietly released at the end of last month – allows you to see how much you’ve spent on Steam in total. According to the tool’s description, the total shown is “a sum of external funds that have been applied to your Steam account, either to purchase games on the Steam store or fund your Steam Wallet.”
Source: IGN.com Steam Adds Option to See How Much You've Spent On All Your Games