sell for a song
1. To be available for purchase at a very (and perhaps surprisingly) low price. This computer used to be nearly $2,000 when it debuted a year ago, but it's selling for a song now.
2. To sell something a very (and perhaps surprisingly) low price. In this usage, a noun or pronoun is used between "sell" and "for a song." The company is having a liquidation sale at the moment, and they're selling all their inventory for a song.
sell something for a song
Fig. to sell something for very little money. (As in trading something of value for the singing of a song.) I had to sell my car for a song because I needed the money in a hurry. I have two geometry books and I would sell one of them for a song.