Information about network egress pricing would be found in the relevant Compute Engine
documentation. The APIs themselves are generally billed in terms of number of operations and storage quota only. Since the Cloud Datastore is accessed via HTTP REST API, you will probably be charged some egress on the Compute Engine side however this may fall under "Egress to a different Google Cloud service within the same region" depending on the location of your instances. It may be worthwhile to open a case with
billing support about this type of question.