What is the cost of additional storage on the Apple One Family plan?
The Family plan comes with 200GB. What are the available options for increasing the storage on this plan without moving to Premier?
You can add an individual subscription for 2TB storage, as described here:
Upgrade your iCloud storage plan - Apple Support
You will then have 2TB plus the 200GB from Apple One - see:
What happens to your iCloud storage plan when you sign up for Apple One - Apple Support