This Gizmon iCA iPhone camera casing might be the ultimate one that I have seen. Made of 32 polycarbonate parts, the casing can mount conversion lens (with additional lenses) and has a working shutter button (volume up button).
What’s more, the casing also has additional pancake lens (of course not the real one) that makes it more realistic. There are also 2 strap holes for you to install a neckstrap and a tripod hole for you to mount onto a tripod.
To aid you, the lens has a mirror for your self portrait. The casing comes in black, white and “retrotastic” orange.
The iCA alone cost US$65 and the strap cost US$30. If you need those extra lenses, it will cost you US$45 for either the macro or fisheye.