Today, it is far less, because of economies of scale and deals between providers at all levels.

Actually, that's never been true. The cost per bit transferred (or even per terabyte) is extremely difficult to measure; it's a tiny, tiny addition to the power bill for the routers. That's it.

What's expensive is building the link and buying the router in the first place. That can cost a hell of a lot of money. But once the link is established, it costs the same to run at 100% as it does at 0%. There's essentially zero incremental cost for usage.

So, everything comes down to peak usage. It's like roads: you don't build new roads because of how much total use they get, but because of how much peak use they get. Only the peak times matter for sizing a road, and sizing Internet connections is exactly the same.

If you're using your connection heavily at 2AM, when the pipes are empty, it costs your ISP literally zero dollars. Off peak usage doesn't matter at all.

In a sense, on-peak usage doesn't cost them anything either, except: when the network is using too much data at peak hours, they may have to set up expensive new Internet connections.

So this is why caps are such absolute bullshit: they are charging you for using up something that's not scarce except for a few hours a day. If their backend is saturated, it's their fault for not provisioning enough to honor the promises they made. It's not your fault for using it: if you've got a 50/5 connection, you should be able to use it 24x7x365. If there's not enough bandwidth to do that, they lied about what you were buying.

In essence, they're trying to cheap out on their infrastructure, and then monetize something that doesn't cost them anything. Only a monopolist could get away with that kind of abuse; if you could realistically switch providers, there's no way they could pull that shit.