Sprint is offering a $200 mastercard gift card when you add a line to the BYOD plans. This even works with the previously free plans (if you got the free sprint plan for one year, you can add another line for free for the remainder of the plan year and get the gift card free). Need to bring your own device but works with any plan.
YMMV - The $200 Prepaid Master Card is available for existing customers and on adding a new line. The offer is for selected users only. But worth a try!
Extra line must be active for 60 days to receive the gift card. Recommend doing this over the phone to ensure you get the free plan (if you already have the free plan and are adding a line to that account). Link is more for adding a paid line, if you don't have the free plan.