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?
Electric tooth brush. Learn to use it properly. Combine with flossing, save a lot on dental issues.
Arm sleeves (Korea brand from ebay) for driving. Keep my arms from turning into toast these days, you could also wet them to enhance cooling