Want to get your hands on the new iPad Air, but don’t have the cash? Well Wal-Mart is launching a special campaign that will pay customers up to $300 for their old iPads so that they can buy the new iPad Air (which they’ll be selling for $479) when it hits shelves on Nov. 1. Starting this week, you can bring in your old iPad for an in-store appraisal and receive a gift card that can be used to the purchase of the new gadget. Of course, there’s nothing preventing you from using the store credit to buy another, equally awesome Walmart product …