I bought it for a song

I bought it for a song is an old expression used when you want to say you bought something for a cheap price. Buying something for a song means that instead of using to money to purchase something, you instead traded it for a song, such as by playing them a song, singing them a song or use it figuratively as a way to say you traded this item for a favor you have done for them. It is another way of saying you didnt pay for this item, but traded something in return.