I've never been offered a better deal with Comcast/Xfinity for using their modems or hardware. Renting their stuff is $10/mo and a modem is $100, last I bought something like 5 years ago now, for a higher end one that supports gigabit service.

So, $100 or pay $10/mo forever, and over the past 5 years that $10 would be $600, or $500 saved by buying my own modem.