HandHeldItems has some cheap iPhone 5 cases. (Reportedly up to 83% off.)
Now, take an extra 20% off via coupon code "20iphone5cases".
Even better, most cases come bundled with a $1 HandHeldItems Credit (whatever the heck that is, but savings is savings!).
Shipping starts at $2.85, although many items receive free shipping. Coupon expires December 31.
A great deal is the HHI X-Factor Two-Tone Skin Case for iPhone 5 (linked above) in seven colors (Clear pictured) bundled with a $1 HandHeldItems credit for $3.18 but only $2.54 after using the coupon. With free shipping, that's $17 off list and the lowest total price out there.