If you are very lucky you get 10% of the price the publisher sells to the retailer. This tends to happen only after you have sold 2000 copies. Before then it can be just 7.5%.
Standard discounts are 35% to bookshops but supermarkets and Amazon expect 60%, leaving very little profit margin for the publisher and low royalties for the writer. However, supermarkets increase sales, so it may be worth it for writers and their publishers. Amazon is a great equalizer. Books that don’t have a chance in getting into a bookshop appear there and are sometimes linked to books which sell very well.
My most lucrative book?
That has to be the “World of French Revision” series which has earned me £6,000. The best paid work, though, is the case study I did for NAWE. £100 for an hour’s work.
One of the nice things, though, is that you carry on earning years after you’ve written the books. And as I tend to go on holiday in September and as royalties are often paid at the end of September, I can feel quite smug as I sit and sunbathe.