From the a podcast titled “The Upside of Quitting” on Freakononmics.com …

“To help us understand quitting, we look at a couple of key economic concepts in this episode: sunk cost and opportunity cost. Sunk cost is about the past – it’s the time or money or sweat equity you’ve put into a job or relationship or a project, and which makes quitting hard. Opportunity cost is about the future. It means that for every hour or dollar you spend on one thing, you’re giving up the opportunity to spend that hour or dollar on something else – something that might make your life better. If only you weren’t so worried about the sunk cost. If only you could …. quit.”