DDoS Price Responsibility?

One of the features of AWS Shield Advanced is DDoS cost protection: AWS Shield Advanced comes with DDoS cost protection to safeguard against scaling charges resulting from DDoS-related usage spikes on protected EC2, ELB, CloudFront, Global Accelerator, and Route 53 resources. If any of these protected resources scale up in response to a DDoS attack, you can request Shield Advanced service credits through your regular AWS Support channel.

You can learn more about the features here: https://aws.amazon.com/shield/features/

I'd also recommend reviewing the following AWS Best Practices for DDoS Resiliency documentation, regardless of opting into AWS Shield Advanced or not: https://docs.aws.amazon.com/whitepapers/latest/aws-best-practices-ddos-resiliency/welcome.html