Iirc it was something small like $5 or $15 a month... I really only did it for the better hardware and software.
I went in and the unlimited plan was about $15 less per month using their modem/router than my own (which I already had), plus the router was free (I'm not paying a monthly equipment "rental" fee).
One annoyance was that their router didn't allow spaces in the WiFi password, so I had to reconfigure all my devices.
I could set up the router in bridge mode where it acts like a dumb modem and continue to use my own router, but I have not bothered with that.
They also force you to share your cable/wifi connection with other Xfinity users who are near you. Buying your own router and modem is a much better deal.
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.
That is, as long as I stay on top of it. Every 3 months like clockwork, they "forget" that I'm not renting their equipment and start billing me for it. I have to call them up and remind them.