So you’re blaming the customer???
We experienced the same issue. We ran our app in Beta for months and carefully observed and modeled our business plan based on the usage reports and ACTUAL REAL MONEY BILLS from Google. We did the correct calculations to extrapolate the bills.
Then, one month after we went live, Firebase fixed their “under billing bug” and our cost went up 800%. Had the “bug” not existed during Beta we would have discovered the issue months in advance and had many options: Change app architecture; optimize database interactions, or switch to a different platform.
Firebase’s price chart explicitly says $1 per Gigabyte of data downloaded from the Realtime Database. It does NOT say anything about SSL overhead. There was no way Firebase’s Customers should have been responsible for predicting this.