Which 2012 iMac?
Looks like it varies from $400 to $800
Just give Apple a call.
Aside form that you can easily do it yourself. The panel is not fused to the glass, so it just requires a suction cup to pull it off and you can buy a new one somewhere else.
check out for apple parts at http://www.thebookyard.com/apple_imac_parts.php