While AT&T has been charging $249.99 for the handset when bought with a new two-year contract, for the same thing Sprint used to want $299.99. Starting yesterday, however, that has been lowered to $249.99, effectively matching AT&T's pricing.
If you've ordered a Nexus 6 from Sprint earlier, and paid the higher price, you don't need to worry. Apparently the carrier is set on making things right, and as such will give you the $50 difference as a credit to your bill. You should receive an SMS with the news at some point.
This information comes from a leaked Sprint document which you can see in the image above. Also from that screenshot we find out that the no-contract price for the Nexus 6 has changed too, now being $648 (down from $696), and as a consequence the Easy Pay monthly payments will also change.
The Nexus 6 isn't cheap even at $249.99, compared to some other handsets, but it's now more affordable than before - at least at Sprint.