Half-Life 2: Lost Coast costs $10 by itself, but it comes free with a lot of their packages containing Half-Life 2. In fact, I don't think it's even possible to purchase Half-Life 2 through Steam now without it.
I think got it for free three times, once with Half-Life 2, again with Half-Life 2: Episode 1, and again with the Orange Box.
There might have been some earlier boxed copies that didn't come with it, but other than that pretty much anyone who has Half-Life 2 already has Lost Coast. Personally I don't know anyone who has had to pay for it.
EDIT:
I checked just now, and apparently you can't purchase it by itself at all so I guess the $10 price on it doesn't mean anything for all intents and purposes.
I guess the only reason they put the $10 on there is because that's the price of the cheapest package that has it. It doesn't actually figure into the price of any packages containing it, so it's pretty much free.