I know that Norton is currently $10 for Prime Day but this is the cheapest this has been for a while and is my preferred AV, next cheapest I could find was on Amazon for $19.99.
Make sure you're not logged into your Bitdefender central account when buying otherwise, the price is $19.99. You can sign up using your normal email and they'll just email you the key.
Mydealz link
Dealabs link
Upucuza.com link