From the FAQ - https://aws.amazon.com/workdocs/faq/
Additional storage: your business receives 1 TB of storage capacity for each active user account. Administrators can allocate this storage across users by setting individual storage limits in the WorkDocs administrator dashboard. Your business will be charged once a user’s amount of storage utilized exceeds 1 TB for that user’s account. Additional storage is charged on a pay-as-you-go basis.
From your example - you've got 2 users so you've 2TB of storage included in the monthly fee. So no additional storage costs for a 1.5 and a .5 TB usage. You can set limits (when you activate a user, the default is 50GB). If you did go above that 2TB, S3 rates apply.
WorkDocs does allow unlimited versioning. When I've looked at versioning of files in WorkDocs in the console, standard S3 rules apply i.e. multiple versions accrue storage ... so a 128MB file that gets updated to be a 256 MB file would show 384MB of storage. In your example - 3 x 10GB versions would report as 30GB of storage used. Likewise, if a user tries to update a document that would take them over their admin defined storage limit - they'll be blocked. And just in case you're wondering - I can't see/find a method of removing/deleting versions of files: so copy most recent to a new file, delete old to remove an iterated file. How that is billed will be dependent on how much storage is available across all the users - going back to 1TB of storage available per active user.
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