"Data transferred "in" to and "out" from Amazon EC2, Amazon RDS, Amazon Redshift, Amazon DynamoDB Accelerator (DAX), and Amazon ElastiCache instances, Elastic Network Interfaces or VPC Peering connections across Availability Zones in the same AWS Region is charged at $0.01/GB in each direction."
You can peer two vpc's and as long as you are transferring within the same (real) AZ, it's free: https://aws.amazon.com/about-aws/whats-new/2021/05/amazon-vp...
Even peered VPC's only pay "normal" prices: https://aws.amazon.com/ec2/pricing/on-demand/#Data_Transfer
"Data transferred "in" to and "out" from Amazon EC2, Amazon RDS, Amazon Redshift, Amazon DynamoDB Accelerator (DAX), and Amazon ElastiCache instances, Elastic Network Interfaces or VPC Peering connections across Availability Zones in the same AWS Region is charged at $0.01/GB in each direction."