I don't think I'd be able to get Orange Box anywhere cheaper than what I paid for it by digital download and if you sold your physical copy they would still need a log-in and password to play the games. I think it's possible to create a Steam account without buying games, for trying demos etc. but I don't know if that means you would be able to play a 2nd-hand game through that account. I suspect not.
Unfortunately, if you buy a game on Steam, it's actually a "subscription" which cannot be transferred. Not only you cannot resell your subscription, it almost always cost more when stores get their retail copies and sell at a discount.
Orange Box is currently $50 on steam (was $45 on pre-order), and will probably stay at that price for maybe 6 months or longer. But as others found, retail copies can be had for around or even less than $40 after tax.
The benefit is that you get it right away, assuming your connection is fast enough to download multi-GB games in hours instead of days. (Some games can be playable once the basic app and the first levels are downloaded though).