Sub $150 Docsis 3.1 is still a ways away.

The modem is what actually lets you use the signal from Xfinity to connect to the internet. The wifi router allows you to use wifi to be able to access the internet instead of having to be connected directly to the modem or wifi router.

You really only need Docsis 3.1 modems with Xfinity if you have internet speeds beyond around 600 Mbps.

Buy yourself a DOCSIS 3.0 modem as there are so many cheaper options

Modem https://www.amazon.com/MOTOROLA-Certified-Comcast-Spectrum-BrightHouse/dp/B01A1E6BA2/

Router https://www.amazon.com/TP-Link-AC1200-Smart-WiFi-Router/dp/B07N1L5HX1/