I think my bucket seat upgrade qualifies, IIRC the seats were only about $60 at the wrecking yard, plus a few bucks for the hardware to build the new mounts. They came from a Wbody vehicle, a '97 Cutlass? I think?
Not sure if this other upgrade qualifies, I had purchased a New Relics electric window conversion kit for the Caballero, but used some old OEM motors from the wrecking yard. Later, the driver side window on the El Camino quit, so I used a motor from the conversion kit to replace it. Free, since it was originally purchased for a different project.