This is a bit random, but do publishers like Penguin move a lot of classics each year? Middlemarch’s rank in the Amazon USA list would seem to indicate it sells around ten copies a day. Does that seem reasonable? I thought it seemed a bit low, but my opinion is truly random. There are >12 different paperback editions, so that would seem to argue for popularity…
My information comes from 20 years ago:
Before the 21st Century book sales were tracked by the New York Times (and others) by surveying various booksellers around the country. The books were then ranked, but sales numbers were unknown except to the publishers who often did not share that info. In 2001 Bookscan was introduced which counted book sales based on the ISBNs scanned in at point-of-sale. In a surprise, it became apparent that the classics sold far better than had been thought and and that led to a boom in classics publishing. I’m told the introduction of Soundscan led to a similar boom in country western music.