Let's settle this. Go onto steam, right click Lost Coast, select update news. Click very bottom title that says 'HL2: Lost Coast Avaliable Now'. Read what it says in the first line.

Half-Life 2: Lost Coast is now available for play for all Half-Life 2 owners.
Also consider the wikipedia entry on Lost Coast. In the first paragraph these lines can be read:

Lost Coast was released on October 27, 2005 as a free download to all purchasers of Half-Life 2. Persons gifted Half-Life 2 are not entitled to it.
Oric, consider yourself pwned.