Assuming Print on Demand w/ amazon. He's not doing that but we'll just assume to keep it easy. And list price of $24
Amazon takes $5 to print Amazon takes $9.60 royalty
That leaves $9.40 or 40% available.
Of that Macmillan takes their cut to cover editing, covers, advertising, etc.
And what is left is probably $2.40 for each sale.
For Self published folks (going eBooks this time). Assuming you list for $10 and get a 70% royalty from amazon.
You'll earn $7 for each sale.
But, then once you factor in advertising (on amazon which goes to amazon), your earnings for each book drop to maybe a 5-30% royalty range. And that assumes you can sell your book for $10, which most self published cannot.
I have friends who sell $10k worth of eBooks/mo who make $2k/mo. So 80% goes to amazon and 20% goes to them.