If you're using it for audio and automotive stuff, you can get by with a really, really basic unit.

EDIT: Spectrum analyzer on the cheap, haven't tried this myself https://www.youtube.com/watch?v=AQJEYeVR_ME

No discussion of cheap audio test gear would be complete without mentioning this handy gizmo: https://www.amazon.com/Dayton-Audio-iMM-6-Calibrated-Measurement/dp/B00ADR2B84

$25 to stop guessing and know if there's a missing/overboosted frequency range: https://www.amazon.com/Dayton-Audio-iMM-6-Calibrated-Measurement/dp/B00ADR2B84/

They might need some break-in time, too.