port price - fixed per customer/month, mostly sunk cost on TCL.
national backhaul - slightly marginal, mostly sunk (their own network)
network equipment - slightly marginal, mostly sunk (their own equipment)
systems - fixed
staffing - largely fixed, if your costs are anywhere near linear in data transferred instead of customers on your network, you are doing it wrong.
office - fixed (same as staffing).
marketing - fixed (same as staffing).
So, where's the marginal cost per unit of traffic? Where is the cost that you wouldn't have to pay if you didn't transfer the traffic?
Well as for as the ports are concerned TCL still don't get that free just because they have some LLU gear, there is still a very real price associated with the last mile
National backhaul, sure they have a network which spans the country, but it's not all TCL fiber in the ground, they use other providers for segments.
Network equipment, this needs to scale with demand so there is a direct cost associated with wrangling more traffic through the network.
But to reiterate, a percentage drop in the costs of one small part of the overall cost to deliver the service, will not come across as a major reduction in overall price, especially when there are still 100,000+ customers willing to pay what they do.
You just have to look around at other ISPs to see what they could offer data at lower rates IF they wanted to, however giving something away when people are willing to pay for it is not great business.
If you feel so wronged by the way an ISP runs their operation then your only real choice is to vote with your feet.