I read it in a paper on PageRank 4-5 years ago. Don’t remember keywords that I could use to look it up. I do remember, though, that the paper had a lot of fancy looking math that caused me to believe it one hundred percent.
But the basic point is that the cost of a domain is the only thing supporting the value of an outbound link.
Though I suppose you could say that the Shannon information in the pages containing those links is the real expense.