You don't need your database on a dedicated server. Not sure why the comparison table claims "$30 + DB server" when your web server is a perfectly good server already.
Now everyone can understand the math.
I wonder if the misconception is from relying on managed database services. Maybe instead of installing their own database, they order one from a service provider like AWS or GCP. That would explain how they'd think that a database like Postgres would have to live on a different machine.