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.
Cogent's CEO has already offered to pay outright for the port costs and data-center cross-connects for any upgrades. The point is to prove that the infrastructure cost of the interconnect are not the issue.
Upgrading that interconnect mentioned in the original article seems like just plugging in some $10k-$20k standard hardware and connecting a single extra wire. That kind cost doesn't matter really, it seems that they're hurting their interconnects intentionally for political reasons.