What’s been the best purchase under $100 that has positively impacted your life?

My best purchase under $100 was a book about triathlon training that positively impacted my life.
 
Transcript: "My best purchase under $100 that positively impacted my life has to be not long after I did my first triathlon back in 1998. I bought a book about triathlon training because I wanted to, you know, go deeper into the sport. I had no idea what I was doing, so that book was a pretty good purchase."