Best Buy offers the DGL Group Marvel Case for iPhone 4 / 4S in Thor (pictured), model no. MVL-1004-AMT, for $3.99 with free shipping. (Avengers, model no. MVL-1004-AMG1, and Hulk, model no. MVL-1004-AMH, are also available for the same price via in-store pickup.) That's $15 off list and the lowest total price we could find. It features polycarbonate construction and access to all buttons and ports.

Also available is the Marvel Spider-Man 4 Case for iPhone 4 / 4S for $5.99 with free shipping. That's $14 off list and the lowest total price we could find. It features ABS plastic construction and a grip-enhancing finish.