According to their blog post How Helm Works - Part 1: Networking [0], on the AWS side of things, for each Helm unit, they are provisioning an EC2 instance, an Elastic IP, and a Route 53 config. I suppose the 128GB backup is also part of this. From my perspective, $99/yr is not a bad deal for all this to be automatically managed.
[0] https://blog.thehelm.com/post/how-helm-works-part-1-networki...