Apparently $5, plus $15 spent on gifts. I have Half Life 2 but it doesn't show up in purchase history, so I don't know how I got it or how much I spent.
I have...
1. TF2: I received the boxed game as a gift. Also gave the game as a gift to someone. I'd say it's worth the money, even if I only have 36 hours total on it.
2. Left 4 Dead 2: Purchased for $5 during winter, also bought a second copy to gift. One of my friends wanted us to play it, which was the main reason I got it. We've only played one game. This purchase wasn't worth it.
3. Portal: Downloaded for free during one of Valve's promotions last year. Great value, I wouldn't feel bad having paid $5, $10 at most, as I beat the game in under 5 hours. More than $10, I dunno if that'd be worth it.
4. Counter Strike, Half-Life 2 / Lost Coast: Don't remember how I got these. I know I didn't pay for CS or Lost Coast. I think Half Life 2 was a birthday present. Probably was purchased at MSRP.
I actually have more non-steam games sitting around. Armored Princess, Evil Genius, Trine, Company of Heroes, plus some PS1/2/3 games. On those PC games I think I spent around $20.
I haven't bought games in quite a while due to my backlog, but I also haven't been making any progress since I keep getting sucked back into Civ 4.