Here are 2 extracts from Trello documents about the upcoming changes:
"Who is billable in a Workspace?
Workspace members - Anyone who is a member of the Workspace, including admins."
"Examples
A free Workspace has 2 members. One board in the Workspace has been shared with 8 other people. This is the only board these people can access. This Workspace would be at the 10 collaborator limit for free Workspaces. However, if this Workspace is upgraded to Standard or Premium, it will only be charged for the 2 members."
The second statement (example) suggests the free workspace with 2 members is free but this is contradicted by the first statement?