I don't think it was explicitly mentioned in the keynote, but it looks like they updated pricing: https://firebase.google.com/pricing/ Can't find the old pricing now, but it seems similar, but with less plan types.
If this link is correct the new plans have a changed a bit, but pricing per GB seems to be more or less the same. A few dollars here and there. But this shows that the plans can and will change.
Smaller services like Compose.io (https://www.compose.io/pricing/) and mLab (https://mlab.com/plans/pricing/) seem to be charging around USD 15-18 per GB of DB for MongoDB. So either Firebase is less resource hungry for Google or Google's scale is allowing them to reap better efficiency or Google is subsidising Firebase and giving it almost at cost.