get me an ATT iPhone 5 that is still on the old OS so it can be jailbroken and I will gladly pay $300, heck and I would buy 10 of them right now...
Also the mophie case is $80 and the element case runs another $100 (if you pre-ordered before they were out)
ATT iPhone5 16GB models average at $475-550 for good condition units
http://www.ebay.com/sch/Cell-Phones-Smartphones-/9355/i.html?_from=R40&_nkw=iphone%205%20att%2016gb&_pppn=r1&_dcat=9355&LH_Complete=1
And you said it yourself there. $475-$550 is the right range, knowing ebay/paypal take 10% try to get $500 here. Not $600+ Also used cases don't add too much to the price.