Song是“歌曲”，for a song字面解释是“以一首歌的价格”。这是什么意思呢？请看对话：

    A: Wow, that’s a nice car. It must have burned a hole in your wallet.

    B: Not really. It’s from my cousin. Since he won’t be needing it after accepting a marketing job overseas, he’s willing to let me have it for a song. Just call me lucky! By the way, have you found a flat you want to buy?

    A: Not yet. I should have bought one during the financial crisis.

    B: You could have bought two then for a song and sold one now at a good profit.

    Note: “For a song” means “at a bargain, for much less than it is worth.” This idiom alludes to the pennies given to street singers or to the small cost of sheet music.