Any book I didn't like enough to finish was a total waste of money.
Any book I didn't like enough to keep so I could re-read it I've also considered a waste of money. From experience any unknown book, even by known authors, has about a 90% chance of being a waste of my money. So the real cost of finding the one in ten of the books worth buying is ten times the price of the book. So i'm very price sensitive.