You can buy a pre-order key through a Hookups seller for $10, but you'll likely be waiting a few days. Wait time aside, I encountered no issues buying DMC and Dead Island: Riptide through him.
Well, since the base price is still $50, to get the game for $10 you're first going to need to wait for Capcom to drop the BP, and who knows when that's going to happen -- the game is almost 10 months old but hasn't dropped a cent outside of temporary discounts.
Portal 2 has local co-op. I think one or both of the new Rayman games has it and Trine 2 as well. Maybe Trine 1.
For games like Portal 2 if you want to use 1 player using kbam and another on the controller you will need the pc gaming wiki to find the solution. Some other games may need tweaks as well.