The way I understand it is that for "small runs" (less than 2500 copies), the cost is $.85 per unit per song (under 5 minutes, there's a slight increase per minute after 5). So, assuming a 1000-copy CD run, that's $85 dollars a song or about $1200 for a 12-song CD. Which is steep. Too high, for us, in fact, to add to the already high costs of printing, artwork, etc.
However, that's for a "cover song". Like, if I were to play "Let It Be" on the acoustic guitar and felt like recording it and selling it.