I started doing this after having a child so that we could have kids movies on hand in Jellyfin, but am now having a lot of fun collecting DVDs and Blu Rays. I’ve also toyed with the idea of using ErsatzTV to have channels to bring the kids back to the old days where things weren’t always on demand.
Thrift stores are a good way to find DVDs (usually $2-4 a DVD, slightly more for Blu Ray). Estate sales are typically better (under $1 a DVD/BluRay).
The most expensive part of this so far has been obtaining storage drives.