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?
A sleep mask. I sleep much better now that all the light is blocked out. Also, if I want to, I can sleep in in the mornings when it’s bright outside and the light won’t disturb me.