A lot of factors go into this including total sale price, but look more at interest rates. (firsttimehomefinancing.com)
If your book wound up on the midlist (which by definition most did) then low volume and a small cut of the books total sales price made it financially impossible for authors to write full time as their sole source of income. (publishingperspectives.com)
STCG = Total Sale Price — Cost of acquisition — expenses directly related to sale — cost of improvements. (relakhs.com)