What is the cost of the plan and can you increase storage space?

There isn't a cost for setting up family sharing, though the family organiser takes responsibility for all the family members purchases (if a member has a balance on their account that will be used for his/her purchases, otherwise the organiser's card will be charged ; members can't use their own card, and the organiser can't use a balance). It doesn't give you extra iCloud storage space (nor space on a device or computer)