What's the cheapest way to get 2K $5 promo credit?

I think you had to purchase a 2K title before a certain date to get the credit. I may be wrong, in which case disregard everything.

I think you had to purchase before the 25th to get it. However, it sounds like many of those packs may get added to the Editor's Choice $5 promo list that goes active tomorrow, if you have any of those from orders over the last month or two (source).

I got the Editor's Choice $5 coupon email yesterday from the Bioshock Dual Pack (which is also 5 bucks). If the 2K packs get added to the EC list I guess this is the option for you right now

I've got 2 spare keys for said game, if you'd like to trade, find me on Steam. As for getting the credit, you could buy a cheaper pack (I purchased the Bioshock Bundle for $5) and get the promo. I do not know if there's a cheaper way/game, but that is really cheap. And you could trade the leftover keys afterwards.
EDIT: Yes, it is as said before in this thread, you had to purchase it before December 25th to get the promo. Simmy me forgetting that. So yeah, it's too late to get it now.

All you have to do is find a friend with a time machine and go back in time before the 25th. Then all you have to do is buy a 2K game and on the 25th you're gonna have the credit.