I was curious, too. So, I looked around.
This bag looks a lot like the Ortlieb Commuter-Daypack City. The Ortlieb isn't actually 100% waterproof because of the materials they used. Comparing against other options in the roll top, waterproof, biking backpack category it's priced somewhere in the middle.
I'd say this is more attractive than the Ortlieb, bigger, and supposedly waterproof. Now, you're not wrong, it is an expensive backpack. I'm not going to join this drop but, I see why they're asking for this price.