https://daily.jstor.org/why-cant-we-end-spam-ask-an-economist/

If we follow Rao and Reiley, and assume an average transaction value of $50, then $438,000 represents 8,760 transactions — which, with a conversion rate of 0.00001%, reflects the results of something like 8.7 billion emails. It sounds like a lot of email, but if Krebs’ information is correct, it only take a week for Levashov to generate that kind of volume.