Well not how much you've spent, how much it values your collection. But what's that number based on? I've only bought one full price game in my entire life on Steam. And it was one of my biggest regrets so I'm not doing that again. So right there all the sale prices I'm paying aren't being calculated right? Then there's the case of free games and humble bundles, back when they were awesome. Hell probably a third to half of the games I have in my Steam account either came from Humble bundles or free giveaways.
SteamDB gives you a valuation based on full price. The article describes an entry on Steam's Help page that gives you an accurate number of how much money you paid to Valve, including micro-transactions.