WOW, $13 to $20, thats crazy.
Heck, I stopped by my local Sprint stores (not reapair centers) and asked them, one handed me a failry beat up used back, the other handed me a brand new back. Before you do anything else, just walk into the local Sprint store and tell them the back of your phone got beat up and you cant find a cover anywhere, then ask them if they have any. Its worked twice for me.
I'm trying to figure out if the local Sprint repair center will give me a bunch of backs if I color some of them for them to sell