I was also looking for that, the only piece of info about that was:

Unlike tape and other glacially slow equivalents, we have taken an approach that eliminates the need for a separate retrieval process and provides immediate, low-latency access to your content. Access and management are performed via the same consistent set of APIs used by our other storage classes, with full integration into object lifecycle management so that you can tier cold objects down to optimize your total cost of ownership.

It seems to have the same pricing like the other storage classes: No fees for accessing the files within the same region and the typical bandwidth fees if the backups will be downloaded to somewhere else.

Intuitively there is some cost for retrieval. Otherwise you'd just use the cold storage to store everything.

Like every cloud storage file access costs money, so the operation to access it. But its so minimal its basically non existent for a backup solution.

Nearline and Coldline have a per-byte retrieval cost in addition to the increased-but-still-low cost per operation.

https://cloud.google.com/storage/pricing