HN had threads discussing this concept (in the context of 419 scams, I think); I don't remember how to find them. Those have enormous attrition rate between the initial contact, and the "walk into a bank and wire your life savings" step.
edit: They discussed a Microsoft researcher's article about scam emails (thanks, walrus01):
https://www.microsoft.com/en-us/research/publication/why-do-... ("Why Do Nigerian Scammers Say They are From Nigeria?") [2012]
https://hn.algolia.com/?query=why%20nigerian%20scammers&type...