Been there with a 33M cost estimate [1]. If the requester is reading this -- hold them to it and request a much smaller but still significant amount of records. Use that as the foundation for "how much time it actually takes". Another option is to get clever with helping them with means and modes of review/redaction. This is how I got the email metadata for 6M emails from Houston for example.
Another example from Vermont's governor's office: https://www.muckrock.com/foi/vermont-80/email-metadata-55744...
Happy to help more but I'll probably be unavailable tonight.
[1] https://mchap.io/that-time-the-city-of-seattle-accidentally-...