Apparently Sprint was a bit too proud of the on contract price of their Nexus 6. They originally were selling it at $299.99, which was the highest on contract price for the Nexus 6 from any carrier. Even AT&T was selling the device at just $249.99 on contract.
According to leaked docs, Sprint decided to lower the price to match AT&T. Sprint will even take things a step further in the name of customer satisfaction. They will be retroactively giving early adopters a $50 refund to compensate for the difference.
On top of this, Sprint is even lowering the overall "buyout" cost of the phone to $648, which ends up being one dollar cheaper than the Google Play Store price.
Source: AndroidPolice