A cheap backpack ended up being the most expensive backpack I have ever owned.

In 2004 I was very young and all my income came from summer jobs, so I got a backpack from Walmart. It was one of their nicer models, had a lot of features and looked pretty good. IIRC it cost $20. I had worked all summer to save for an MP3 player, and 2 months after getting that backpack I getting off a bus, when I realized there was already a hole in the bottom of the bag. My MP3 player (a creative zen micro) had slipped out of the bag, and someone had already picked up my MP3 player and walked away with it. Adjusted for inflation I spent over $500 on that MP3 player. Even as an avid backpacker, I have not spent that much on fairly nice packs.

In 2007 I splurged and paid $100 for a backpack from Deuter, and I also felt a lot of guilt as that was a huge amount of money for me at the time for something like a backpack. It's been nearly 20 years, it's not just that the backpack is still working, it still functions virtually like new. None of the seams are stretching, even though it's been through incredible overstuffing and abuse. All of the zippers are smooth as silk, and even the cushioning on the straps and airflow offsets on the back are still supple and supportive.

I spent ~$400 on a Deuter baby/toddler backpack two years ago and it still works. Beautiful thing.

Same, I was gifted a Sandqvist backpack ten years ago. I travel with it as my sole piece of luggage (you can imagine the overstuffing). It has outlived three laptop backpacks that don't go through half as much as it has.