Builders don’t cost you anything, they don’t show up in usage at all. We only record usage (and bill) for apps/machines that you created.
To be super clear: the additional RAM charges you see are due to machines you’ve created.
If you want to reduce the amount you’re being charged for additional RAM, you will need to scale the amount of RAM you’ve configured for your machines (and check your fly.toml to ensure that a subsequent deploy won’t undo any such changes).