> A $35 JanSport that dies in eighteen months: $23 per year. […] A $200 bag that lasts ten years: $20 per year. Already cheaper.

If it is, it isn’t by much. The difference between $200 paid now versus 7 times $35 = $245 over a period of ten years is about 5 years of interest over $200. At 4% interest, that’s about $40.

Don't forget to account for your time buying each new one and writing your name in it. If you don't replace soon enough also account for the lost value of things that fall out the holes.