As we just discovered first hand, all of those impulse buys and thrifty mid-Steam sale deals soon add up, and now you can see just how much you've spent on Steam over the years by checking out this new feature.
It's worth noting that the page doesn't estimate the value of your entire library, as games attached to your account via a redeemed code (such as those titles you may have picked up as part of a bundle) aren't included in the calculation. If you want to know an estimate of the overall worth of your account, the SteamDB Calculator should sort you right out.