Steam is having a sale this weekend including Borderlands: The Pre-Sequel + Season Pass for $30.79 USD, however GameAgent is also having a sale including Borderlands: The Pre-Sequel + Season Pass for $20 AUD (~$14,55 USD).
It will say it's the Mac version but once added to Steam account you can play Mac, PC, or Steam OS version (they also have a Linux version which also works for any system.)
That's a whopping ~87% discount down from the ~$151 AUD full price it costs on Steam normally. Enjoy.
Note: Make sure you select AUD as the currency or you will be charged $20 USD instead. There's a drop down in the cart to select currency. You should see AUD$20.00 displayed when the currency is correctly set.
EDIT: No, I'm wrong. I just didn't read that last line.
Deal is 20 USD, not AUD (which when paying by PayPal comes out as $28.52 AUD)