You're doing it wrong. Call them and tell them you can't afford that much every month and you want to see what they can do to lower your bill. I don't k now about the really basic tier, but you should be able to get 50Mbps for $50-55/mo for a year. Set a calendar reminder, call back before it expires, do it again.
I'd rather go to Jiffy Lube for a rectal exam than talk to a Comcast employee on the phone. 🙂
No no, you're not trying to cancel, just pay less. They're happy to help lower your bill because it makes them look like they retained a customer. My last two calls with them have really been quite pleasant.