Not to be flippant, but how does Wal-Mart do it, if it's so easy to find them for half price?
The $2 seller on Google Shopping sells them bulk ie. unwrapped, loose cards, and charges $6 shipping for an item that can be mailed in an envelope with a first class stamp.
The first three sub-$10 vendors I found on Amazon have terrible feedback about unworking cards, slow/no service, mixed up orders etc.
Value is a relative proposition. You don't have to sell something no one else is selling -- as you said, good luck finding it before Target/Wal-Mart/etc. do and get it on their shelves. You just have to Link hidden: Login to view.
Of course, coming up with a unique selling proposition for something as basic as a memory card may be kind of hard, but that applies to almost anything you can think of. I have a friend who sells phones and accessories at break even and occasionally loss prices, because he makes all of his money on the e-mail list he's built out of customers. So his operation is known for super-cheap retail phones and accessories, but he's bleeding money every time he ships a phone. His approach to selling phones is that it's a relatively cheap way to get qualified leads for his list.
You could sell the cards with a mail-in rebate, and laugh all the way to the bank on the idiots who don't send in the form. Consumer/retail incentive company Parago released a study a few years ago that showed that for some electronics items, rebate redemption rates were sub-10% -- and that the cheaper an item is, the less likely customers are to bother with sending it in.
Just saying that there's more than one way to skin a cat, and that price is not king in all cases.