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.
Also I feel like if you are concerned about forgetting your wifi password you'd probably just keep the one that's written on the device (and which is probably quite a bit more secure than the password you'd come up with yourself).
That's my experience with ISPs in SF. It's clear that many people don't buy Internet access. They buy "WiFi" which is that Xfinity integrated service. The components don't matter.
But I've assisted people who's mental model is simply "Verizon put this box in my home and now I have internet". Who panic when a site doesn't load, and will call the first person they think is responsible for the problem. (typically, the company that gave them internet). Or more commonly nowadays, "my phone is my internet connection" -- and the only thing they think they have the power to do is to wave the phone in the air to find 'more bars'.
I suppose it makes sense from Verizon's (or any ISPs) perspective, and honesty, if you understand how all this works, then you understand how to trivially eliminate the issue, and then of course, you know when and when not to call Verizon with problems. (Of course, it'd be awful nice if they offered 'Shibboleet' [1] service for folks who do undertsand when the problem is between the site and the router.) HOWEVER, it'd be nice if they were more upfront with the disclosure of this password sharing ...