I’m very aware of that tool but it’s far from perfect. I’ve spec’d things out on that then seen very different prices when I actually create things in AWS. In part because the tool doesn’t take some things into account or because sometimes it’s impossible to guess your usage for a new feature.

I don’t believe the VPC was factored in when I used that calculator, even after selecting RDS Proxy.

VPCs are free. Are you talking about data transfer? I know it has options to enter values like amount of data transfer you are planning to do.

I believe the cost was actually a "NAT gateway" attached to the VPC which has a monthly cost of about $30 even if you don't transfer any data over it.