I've got an extra white back cover for the iPhone 4 that I don't need.
It's an aftermarket Chinese import. Brand new, still has protective plastic on it. It's made of glass, the camera lens bezel is metal and has circular machining like OEM. The flash has a diffuser. The brackets that the screws screw into is also made of metal and it has a metal shield on the inside. From what I can tell, it's a pretty good knockoff.
$5+S&H, First Class postage within the USA is probably going to be around $1.50.