Depends on your withdrawal method. I would not leave money sitting in my Upwork account. In the US, I pay $1 to transfer to Paypal and nothing for a direct bank deposit. Overseas freelancers have a different situation.
re: "Correct! I have my account set up for direct to bank. But how does Upwork's withdraw fee change depending on if I withdraw $100 or $1000?"
Crystal: You live in the U.S., and you're withdrawing to a U.S. bank account.
So there is no fee.
Fee is zero no matter how much you withdraw.
There is literally no reason for you to wait for $100 or $1000 or any other amount before you withdraw. If you have money available, just withdraw it.