The bench mark for illegal predatory pricing is when the predatory provider sales a product for less than what it costs them to provide the product.
Comcast could certainly sue Google with an antitrust claim to prevent Goggle becoming a monopoly if Google provides a service service so low in cost (or similar cost with higher speeds) that it could result in Google being the provider of choice by a wide margin. Antitrust laws are there to benefit the consumers. The burden would be on Comcast to show Google's potential monopoly would ultimately not benefit consumers.
With that said....What if an incumbent cable operator had a 15 Mbps tier for $50 per month and a 30 Mbps tier for $60 per month. Would you switch if a new provider came into town with 20 Mbps for $30 per month and uncapped service and still make a very healthy profit?
What if new provider offered merely 10 Mbps for $30 per month, or jaw-dropping 30 Mbps for $30 per month?