Eventually it adds up. I'm selling every card I've obtained and have managed to buy a couple good games. (Keeping my Summer Sale ones though, how dare I not get a stupid badge).
I'll definitely be selling my Summer Sale dupes. I've maxed out my regular Summer Sale badge and still have around $800 to spend (plus the ~$135 currently in my Wallet), so I'll be in for at least another 93 cards plus the remaining several I'll receive from Community's Choice votes. The regular cards will probably be hovering around 10c by the time the Summer Sale is drawing to a close, but that should be enough to pay for the foil badge.