Walmart offers the refurbished Apple iPhone 6 Plus 16GB Smartphone for Sprint (no contract) in Silver, model no. MGCW2LL/A or Space Gray, model no. MGCV2LL/A, for $339 with free shipping. That's $90 under our May mention, $86 less than what you'd pay for a new one, and the lowest total price we've seen for this phone from Sprint.
Note: A 90-day warranty applies, but it's unclear who covers it.