It's not necessarily $20 or less. I tried using the code on one of the sims pack that cost $30, and it worked. Alan Wake is $30, but the code doesn't work on it despite being the same price. I think it only applies to old games. But it worked on shank 2, making the price $4. So I guess some new games work?
I really want shank 2 for $4, but i'd rather give my money to steam. I'll wait for a steam sale on it.