How does billing work for Workspaces that are recreated.

That would be considered 1 WorkSpace for billing purposes. This assumes:

• it's the exact same bundle type, both in terms of hardware as well as in terms of BYOL vs. License Included, plus software. If it's "Standard/License Included/no software", and then deleted and recreated to "Standard/License Included/Office+Trend Micro bundle", that is 2 unique WorkSpaces.
• it's the exact same AD user/same directory. This is based on the Active Directory SID of the user (probably also AD Connector ID, if for some reason you have 2 AD Connectors pointing at the same AD - I'd keep them in the same AD Connector to be safe).