Sprint is quickly taking reins of the prepaid market with the growing expansion of its Common Cents service to another 500 Wal-Mart Stores in Chicago, New Orleans, Cincinatti, Washington, D.C., and more. Common Cents was launched back in May and offers a cheap pay-as-you-go service promising 7 cents per minute or 7 cents per text. You pay via $20 refill cards that are good for 30 days, or $30 cards that are good for 60 days. As a promotion, Common Cents will also offer an additional 200 bonus minutes for domestic use when the phone is filled for the first time.