BuyDig offers the uNu Power DX 2,400mAh Rechargeable Battery Case for iPhone 4 and 4S in Matte Black, model no. DX-05-2300B, for $79.99. Coupon code "IPHONEJUICE" cuts the price to $45. With free shipping, that's the lowest total price we could find by $35. It purports to provide an additional 10 hours of talk time or 8 hours of additional web-browsing