For me, it was a pair of merrell shoes on sale. They have lasted 3+ years, have great grip in the snow and ice, and so comfortable, my feet don’t hurt after wearing all day.
Never cheap out on anything that comes between you and the ground; shoes, tires, a bed.
What have been some of your good purchases under $100?
Both my awesome used thinkpad ($1200 new in 2019, $100 used with a broken screen in 2022, $40 for replacement screen), and all the nice (nicer than office ones) pens I buy.