Thermal underwear. Entering the 10th year of a 30 euro Helly Hansen long underpants that keeps me warm half the year.

Raspberry Pi 4 that served as my daily driver for around three years.

A couple of dumbbells that got me started with weight training, and kept me going during covid. Together with the basic equipment that I later bought, it saved me hundreds and hundreds of euros in gym memberships.

Best purchase under 10 euros is a simple cube with blank memo notes, that I use for grocery shopping and all kinds of other to do lists.