Yes, you can buy extra storage for each and every single consumer who has passed the storage limit no there Gsuite email attached to their domain.

Available storage picks are 20GB, 50GB, 200GB, 400GB, 1TB, 2TB, 4TB, 8TB, 16TB

For pricing, send an e-mail to hello@onenetservers.net

Read the Difference between G Suite and Google free apps