There is some cost to transfer between ISPs or to higher tier ISPs, though generally that cost is negligible. They always pay for accepting traffic and generally they have agreements that only the difference between out-in is paid. At my current host, the peering cost of data is about 5€/TB outgoing transfer, though recently they removed this altogether and cover it from other profits.

The real world cost of a TB of data is basically the cost of the energy moving it, which is increasingly cheap.