Fig.: to buy or sell something very cheaply

Lucy found a very cute watercolor at the flea market and bought it for a song

Lucy ha trovato un acquerello carinissimo al mercatino delle pulci e l'ha comperato per un tozzo di pane

It was an interesting piece of modern art, but it seems that nobody liked it or wanted it, so I bought it for a song

Si trattava di un interessante pezzo di arte moderna, ma sembra che non piacesse a nessuno e che nessuno lo volesse, perciò me lo sono comperato per due lire

One of the first examples of this expression is from the early 1600s and can be found in Shakespeare's All's Well That Ends Well (Act III, Scene 2): “… I know a man that had this trick of melancholy sold a goodly manor for a song”. At the time, people would sing on the street for pennies, only a few though, since everybody can sing! Buy for a song is therefore closely related to Sing for your supper