You're right to call me out here. Although I have a reasonable idea what it would take technically, I don't actually know the financial impact to Comcast for increasing capacity to L3 or Cogent (not even ballpark). However, I'm taking the L3 article at face value when it says:

    But there are also typically shared costs for networks to 
    interconnect. Each party pays to augment its own network 
    to allow for more traffic exchange (the expense to augment 
    capacity is not significant for either party). And since 
    we often choose to interconnect in a third party data 
    center, the networks usually agree to share the cost of 
    the cross connects by paying for them on an alternating 
    basis.

I take that to mean the CapEx is insignificant and the recurring expenses are shared equitably. What's notable about that is that Comcast isn't even sitting down at the table to negotiate equitable terms for upgraded capacity. They're out and out refusing to upgrade without direct payment.