A song can be worth almost anything. The value is very much dependent on sales. In the US, the average artist earns around $2 per CD sold, 25 cents for each iTunes download, and $0.005 every time a song is streamed digitally. Obviously, each song will be worth a lot more if it receives a lot of sales, downloads, and streams. Some artists have made hundreds of thousands or even millions of dollars from a single song.