for a song

get / buy / sell something for a song (if you buy or sell something for a song, it is very cheap; at a low price, cheaply) — за бесценок; очень дешево; дешевле грибов
This is especially true when the item is normally very expensive.

Example 1: We bought the car for a song.

Example 2: He sold the invention for a song and its buyers were the ones who got rich (Словарь американских идиом)

Frequency of usage: sometimes

Related vocabulary:
[on the cheap]
[on a shoestring]
[задарма]

[for free]