Interactive Brokers Recurring Costs (question)

I believe it's only earned above the first $100k. Accounts below that threshold don't earn interest.

 

Check out this web page: https://www.interactivebrokers.com/en/index.php?f=1595
Select the tab "Interest Paid to You".
Whether IB pays interest to you depends on the currency and the amount you are having in cash in your account. There are currencies where IB charges you interest though.