Pangea Deals via eBay offers the refurbished No-Contract Apple iPhone 6 Plus 16GB Smartphone for AT&T in Space Gray or Silver for $529.99 with free shipping. That's $20 under our July mention and the lowest total price we've seen. (It's also a current low for an in-stock refurbished model by $131.)
Note that no warranty is provided, but it's covered by a 30-day guarantee. Additionally, this item doesn't include headphones, the user manual, or original box.