Under $100 – a Bluetooth receiver so I can use any studio-quality wired headphones or my car stereo with any Bluetooth transmitter (smartphone, computer, etc.). I use this thing every day!

Under $1000 - an e-scooter. It has completely transformed my relationship with my city. I rarely drive a car anywhere anymore. If, god forbid, my e-scooter were to break or get stolen tomorrow, I would immediately purchase a replacement.