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 TempSpike wireless thermometer. I was pretty good at guesstimating cook times for roast chicken based on weight, but this thing gives me perfectly cooked chicken every time.