When buying a Certified Pre-owned device the upgrade discount does not apply.
From the more information section of the device
Note for existing Sprint customers: If you upgrade to a Certified Pre-owned device, your current upgrade eligibility status is not impacted. You pay the rebate-free pre-owned price. When you're ready to make your eligible upgrade to a brand new phone, you'll still be able to benefit from the instant and mail-in rebates.