I did some wheeling and dealing, sold and traded extras and cards from other games. Managed to complete 5 badges so far during the sale, and made a couple of bucks. Sins and LH are tough to complete, 15 cards compared to 8 or so for other games I play.
I've sold most of my cards, but the ones for Endless Space weren't selling, so I decided to buy the rest of the set with the money I made from other cards... and the rewards it gave me sold for more than the price of the 4 cards I bought (I made something like 20 cents after the buying and crafting). Color me surprised. I guess if you really wanted to, you could meta-game the system and craft badges for games with expensive rewards... if you're in to that sort of thing.