Specially L4D2 key or... any kind of Steam Digital Key (not as scan) ? Because I have one for $10-$15 that contains games into (after adding).
This is if you want to have a PoP. If not and the only thing that you want is a L4D2 key, then I can't help you (erm, I can, but Retail L4D2 keys aren't cheap...).