I spend a lot during every Steam sale, it's just to hard to pass up a good deal. I learned to buy everything as gifts, that way if I don't get around to playing it I can gift it to someone else.
Go here to see the cost of all the games you've gotten thru steam
http://www.steamcalculator.com/It show my current value is $1406 but my real cost was probably about 25% of that since I only buy during big sales.