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?
My old Kindle Keyboard (3rd gen kindle), still about $30 on ebay, I believe, is my absolute favorite way to read long-form information, and I can hold it however is comfortable instead of awkwardly avoiding the screen with my thumb. I read a lot more books because of it.