According to their advanced pricing calculator at https://purelymail.com/advancedpricing you can get about 16GB for $10 per year. The price for 1000GB would be $376.65.

    $210.40 for storage, $4.00 in account fees, $112.89 for received email, $2.77 for received email bandwidth (GB), $46.53 for sent emails, $0.00 for sent email bandwidth (GB), and $0.05 in username fees.

"If you use significantly more than $10 a year in resources, you may need to switch to advanced pricing"