I believe that much of the difference between Comcast and GM are because of number of customers who are revenue generators and the barriers to competition that Comcast would have compared with GM. While there might be many loyal GM purchasers, (I don't know that metric), Comcast does have competition for cable TV (e.g., many who are "cutting the cord") but not so much as an Internet provider, AFAIK.

GM's high revenue/profit vehicles require a lot of gas compared with the lower revenue/profit vehicles and thus the projected cost of gasoline probably is part of the GM valuation. For now, and for the future, thanks to fracking and (relatively) low demand from China, it appears as if GM should be having increased profits.

Regarding, Tesla's valuation, there must be some metric for customer acquisition.