Most upvoted comment
Reddit, What is One Product Under $20 That You Recommend Everyone To Buy?(r/AskReddit)
A fire extinguisher. You probably won’t ever use it, but if you do need it someday, it will be well worth the 20 bucks.
EDIT: Yes, you can get a decent one for the home for under $20.
EDIT PART 2: Aaaaahhhhh thanks for the gold! You just made my night! 😀