Cloudflare $10 for 1 YubiKey.
Log into your Cloudflare account and navigate to the dashboard to claim. Coupon code will be emailed to you from Yubico in 1-3 days. Deal valid for up to 10 keys per account.
YubiKey 5 NFC - $10 (regular price $45)
YubiKey 5C NFC - $11.60 (regular price $55)