How should we approach a TCO (Total Cost of Ownership) analysis for migrating from Azure to GCP?

have Azure infrastructure and usage metrics across more than 20 subscriptions. I’ve been asked to perform a TCO analysis for migrating these workloads to GCP.

The challenge is that there is no clear architecture documentation, no complete billing invoices, and only partial information available for only a few subscriptions.

My questions are:

  • Is it practical to produce a reliable TCO analysis with this level of information?

  • What is the right way to approach this situation?

  • How should I estimate the migration timeline to GCP?

  • Is it better to request more information, or should I proceed by making reasonable assumptions, mapping Azure services to equivalent GCP services, and estimating GCP costs based on those assumptions?

This is a mid-sized enterprise environment.

Any guidance would be appreciated.