Avoid high daily charges with Cloud SQL

Question

So I am in a developing mode, and within 10 days of developing, I had a 300$ bill to Google.

What I did :

1. Created a test table, and added records to it - text and numbers.
2. Performed multiple queries, from my Mac, I would say 100 a day for 6 days.
3. Have a table with around 100k rows and 6 columns.

This is what I do for every query (does reconnecting for every query cost more?) :

engine = sqlalchemy.create_engine('postgresql://' + username + ':' + password + '@' + host + ':' + port + '/' + database)
df = pd.read_sql_query(current_query, engine)

4. I am writing around 2000 rows at a time ( only me), and I was sometimes reading thousands of rows - for the development.

You can see my charges and

5. CPU show 1.4% constantly for 24 hours, but I do not use it all this time.

I can see a daily charge of 9$ for RAM and 8$ for CPU. The charge is constant and daily, so it seems it is not relate to my action.

I talked with Google, and they could not explain the charge, nor you can see this on the Console.

What can I do to understand exactly for what action they charged me ? it seems like a daily fixed amount.

If this is how Google charge, it is scary to think how much would it cost with 10k users (or more)