Instead of a laser gauge, you could try using a camera hooked up with some edge detection software and use that to estimate the width. It probably wouldn't be as accurate, but it would definitely be cheaper.
At some point dust in the air will be a limiting factor.
As long as your material is consistent (no bubbles, shiny spots, etc.), You should be able to build a very accurate sensor this way.
1: https://pdf.sciencedirectassets.com/274145/1-s2.0-S175161611...
Though your idea of a light source and adc could work well if you put it in a darkened enclosure. It’d probably require a bit more hardware design or tuning but the software / microcontroller side would b simpler. A RPi PiCam with a macro lens would work well too, but the reverse with less hardware tweaking and more software.
They use a laser pointer to measure width of a hair.