Wow, that's a terrible price from my perspective. Anyway, here you go.
Do I have to have an Internet Connection to play Half Life 2: The short answer is Yes. If you don't have any kind of Internet connection, you cannot play Half Life 2. Steam requires online authentication of your Half Life 2 CD Key and it also needs to download additional files to activate and complete the Half Life 2 install before it can be played. There is no legal way around this - the CD Key must be authenticated online, and sharing Steam cached files with your friends is illegal. However once you've registered your CD Key and updated Half Life 2 you can play it offline at any time without having to have an active Internet connection - see below.
Playing Half Life 2 offline: Once installed, verified and updated online, you can play Half Life 2 without having to be connected to the Internet. To do this, follow the Steam Configuration instructions further below. Then whenever you want to play offline, make sure you don't have an active Internet connection and launch Half Life 2. When prompted, select the 'Play in Offline Mode' option and the game will load and play offline. For more detailed information, including playing offline on other machines, check this: Steam Article.
http://steampowered.custhelp.com/cgi...hp?p_faqid=213