The answer is going to vary tremendously depending on your budget, the composer's experience, the length of the film, and what deal you can work out. The composer that I work with charge me anywhere from $200-400 per short film, but the length of the film varies greatly.
As far as song licensing, that also depends. If you want to use a Beatles song you're probably not gonna be able to get that for free, but if it's an independent band then that's a different story. I've gotten permission to use some great songs for free from indie artists by simply contacting them and asking for permission. You just have to try it out.
You can license music for soundtrack very inexpensively from Magnatune. There is a huge selection of music of all kinds, and the quality is very good for some items. In addition, you can preview all of the material online and the license process is automated. Check out Shira Kammen's stuff, it is great for soundtracks, and there is some industrial stuff as well for more gritty scenes, but there is lots to choose from.
