What's the cheapest thing you had bought that by the end of its usefulness made you say "wow, this was worth a lot more than what I've paid for!"
Mine... My Xbox 360 slim only costed 129 euro back in 2012 and to this day still work like brand new, you would think that the disc drive would stop working but no. Never had the need of open it or clean it's insides. Still great, I just don't use it anymore since I feel it's outdated and loading speeds are better nowadays.
Your question is a little too narrow for my tastes. Some of the best cheap purchases that I've made are things that I still have now. They haven't worn out or broken or become useless.
In 2001 I bought a new cast iron fry pan for $20 that I use on regular basis and it's marvelous. Around the same time, I bought some used silverware from a thrift store and I still use that everyday.
Two other purchases that go back more than 20 years are my pocket knife and my 1/4 inch hex driver, both of which I bought new but were not particularly expensive and they are working wonderfully to this very day.
FWIW, we cleaned out one of the out buildings on my family's rural property which was built in 1903. I found some rusty cast iron pans. Reconditioned them and use them all the time. They are at least 115 years old.