Even if the so called Tier 1 carriers are losing their prominence due to donut peering of smaller operators and large content providers building their own networks, getting settlement free peering with the big ones remains a challenge. Settlement free peering itself isn't an exclusive club, but being an operator with significant market power is.
While the majority of settlement free peering agreements are informal, studies show that about 10% are more formal with written contracts and conditions.
Even with public peering fabric which is what I think you are talking about and that I've used LINKS, NYIIX and AMS-IX etc,. you still have to backhaul that traffic yourself, its not making a significant dent in Tier dominance.
What studies have shown that there are contractual agreements for settlement free peering? Citation? Links? I would be interested in seeing that. My source was from experience working in the industry.
Transport, crossconnects and port costs apply regardless if you use a public IX or do private interconnects.
Here's a link to the study about settlement free peering: https://www.pch.net/resources/Papers/peering-survey/PCH-Peer...
I had the percentage wrong, or then I may recall a more recent study. This one is from 2011. Of course it depends greatly upon who you peer with, as to whether a written contract is needed or not. As such the sampling may skew the results.
-Bill