Right now, you can grab a refurbished unlocked Space Grey 64GB iPhone 6 for just $399 at eBay. You're looking at model number a1549 here, which is an unlocked model and should work on both CDMA and GSM carriers. The seller, pangea_deals has over 36,000 ratings and 99% of them are positive.
The listing does note there may be minor scratches or blemishes, but should you find yourself in the market for a 64GB iPhone 6, this may be the deal for you. If interested, be sure to act quick as we are unsure how long this will last.