I just did my aunt and uncle's taxes, zero income for both, joint filing, it's asking me to pay $21 per return, is this correct?

TurboTax Online Deluxe is $21 per return. So, if they need to use deluxe, that's the correct price.

 

Please see this TurboTax FAQ for more information on who can use Free:

Who can file for free using TurboTax?