Thanks for the tip, in Michigan and Texas it does appears that you can pay $30/mo for unlimited add-on ... but only if you buy your own cable modem instead of using theirs. This was listed plainly on the new Broadband Facts / "nutrition facts"...I hadn't seen it mentioned before!
I couldn't find anything about a $50 add-on in my states, and I haven't seen any other options for customers who are using the Xfinity-provided modem.
For others wondering why people should care, my biggest concern about this isn't the cost. It's two points:
1) Historically this wasn't ever presented to customers before they signed up for service. All you saw was "1000Gbps for $80/month!", Xfinity wouldn't tell you anything about a data cap until you hit 90% of it and they texted you out of the blue saying "Hey your $80/mo is going to be much higher this month!". I have much less issue (still some, but much less) with it if it's clearly shown up-front! Thanks to the FCC, it now is shown up-front.
2) There's no way to dispute the charges if you believe their "meter" is wrong. Every other utility has meters which are subject to your states "Weights and Measures" department -- if you have reason to believe your utility meter is malfunctioning, you can have the state come out inspect the meter and determine if it's out-of-spec. But Xfinity consumers have little[0] to no recourse if Xfinity errs in their "metering". I don't think that type of business practice should be allowed without being subject to some kind of real oversight.
0: https://arstechnica.com/information-technology/2015/12/comca...