I doubt the price is anywhere near $100, but the Ortleib backpacks, if made as well as their panniers, will be a completely waterproof bag for decades of hard use. I have their front paniers. Got them in 1999. 10 years of commuting several days/week year 'round and shopping, I "retired" them and bought new ones. (Actually just bought new replacement hardware and set them up to be rear panniers, not fronts. Became my farmer's market and touring pannier should I do another.

Only repair the bags themselves have ever needed was for a 1" slice made by a very sharp knife. Standard rafting repair and it is like new. I can fill those now 20 year old bags with water at the well and carry the water home. (Haven't but simply because I've never needed to.)

Ben