Specially Composed Music
Generally speaking, commissioning a piece of specially composed music will cost more than licencing existing music as you'll be paying the composer for their time and creative input.
But you'll be getting music that is specifically tailored for your production and, if your programme features a lot of music, then specially composed music can be good value when compared to the equivalent duration of library music.
A Producer's view
Steve Greenwood Series Producer BBC Natural History Unit talks about using specially composed music in the 'Expedition' series for BBC1