Here are some examples of how it might be used in a sentence:
* "The antique furniture was going for a song at the flea market." (Meaning the furniture was very cheap, even though it was old and valuable.)
* "After the store closed, they were selling all their inventory for a song." (Meaning they were selling everything at a very low price, likely because they wanted to clear out the stock quickly.)
* "I got this vintage dress for a song at a thrift store." (Meaning the dress was a very good deal, despite its age.)
The idiom suggests that something is being sold at a price so low, it's almost like you're getting it for free, like someone is singing a song for you instead of charging you money.