If you're on a cheaper lower speed subscription, you can often find compatible modems at thrift stores for a couple dollars. People upgrade to faster tiers and unload their old perfectly serviceable equipment good for a couple hundred megabits - fine for most needs.

Wow, what a deal. Last I looked it was $5/mo. Spectrum doesn't give you any discount at all.

Still I thought a good DOCSIS 3.1 modem would be a few hundred.

I did this recently and found out Comcast considers some security feature that runs only on their hardware to be part of the bundle they sold us.

So, bringing your own modem gets rid of the rental fee, but requires moving to a different plan without the security feature bundled. This is of course more expensive, almost entirely negating the savings of bringing your own network equipment (I think our net savings is $5/month, which means its going to be a couple years to pay back the modem cost).