See below from the VPC FAQ
Below is discussing Endpoint ENI but concept is the same for Lambda also because Lambda also spins up an ENI.
Q. Who pays the data transfer costs for the traffic going via the interface-based VPC endpoint?
The concept of data transfer costs is similar to that of data transfer costs for EC2 instances. Since an interface-based VPC endpoint is an ENI in the subnet, data transfer charges depend on the source of the traffic. If the traffic to this interface is coming from a resource across AZ, EC2 cross-AZ data transfer charges apply to the consumer end. Customers in the consumer VPC can use AZ-specific DNS endpoint to make sure the traffic stays within the same AZ if they have provisioned each AZ available in their account.
Clarifying MSK data transfer pricing within region or AZAccepted Answerasked 3 years ago
What are the data transfer costs between Lambda and Fargate?Accepted Answerasked 5 months ago
From where data transfer going to be performedasked 4 months ago
How to determine the root cause of Data Transfer - regional data transfer - in/out/between EC2 AZs or using elastic IPs or ELB fees?Accepted Answerasked 9 months ago
How to reduce Data Transfer Cost in AWSasked 9 days ago
How does data transfer pricing work between lambda function urls and cloudfront?asked 5 months ago
data transfer cost between two AZs in the same VPC same accountAccepted Answerasked 2 years ago
Alternatives to minimize data transfer costs for backup solutionsAccepted Answerasked 3 years ago
By using ec2 to Eks cluster creation data transfer consumptionasked 4 months ago
Same VPC/region data transfer cost from lambda to ec2asked a month ago