For a song 作何解释？

mcchan

(1) for a song 即very cheaply，意为「以贱价……」，例如﹕

Brad's car is practically new, but he says that if someone will pay cash
he's willing to let it go for a song.
布雷德的汽车与新的差不太多，但他表示，若有人支付现金，他情愿以贱价售出。
(Modern American Colloquialisms)

(2) for a song 还常与buy、get、sell等动词连用，例如﹕

They bought the flat for a song and sold it a few years later at a good
profit.
他们以极低的价格买下这层楼，几年后沽出，赚到一大笔钱。

(3) I got this LV bag for a song at a thrift shop.

我在一间旧货廉售店，以低价购得这个LV手袋。
(注﹕thrift shop乃美国专收旧衣物等捐赠物品的店铺，收益多用作福利事业。)

(4) He sold the invention for a song and its buyers were the ones who got
rich.
他们将该项发明平价转让，买入者因而发达。

(5) 亦可用for a mere song，或for an old song，来代替for a song，意思不变，
例如﹕All of these lovely things went for a mere song.
所有这些珍品几乎等于白白送掉。

(6) 旧时艺人在酒馆卖唱，赏钱微薄，故有for a song的说法。