> 1GB storage / day can quickly add up, for example
Are you sure? To me that sounds like it'll let you store 1 GB indefinitely, or 0.5 GB once a day for 12 hours; not 1 GB for a day on day 1, 2 GB for a day on day 2, etc...
"1GB indefinitely" is the case I think. The legacy GAE datastore allows you to store 1G of data for free over the course of a month. Datastore I/O is charged separately after free limits are exceeded. If I understand correctly the Cloud Datastore is the legacy GAE datastore with a different API (no NDB) and the free tier for both is essentially the same.
