Fivetran charges by "monthly active rows", which quickly adds up when you have hundreds of millions to billions of rows that are constantly changing.

https://fivetran.com/docs/usage-based-pricing

yep, and Notion's data model is really bad for this pricing. Almost every line you type is a "block" which is a new row in their database.

They’re likely paying for egress from the databases as well.

DBA salaries, maybe?

Maybe cloud hosted