There really isn't a set answer for this.
There are three questions you have to ask when deciding how to price something:
1) How much do I want to make off of this song? (Of course we'd all like to make $100,000. Be realistic.)
2) How big is the person who wants to buy it, and what is their budget? (There's nothing wrong with working with your clients in a fair way to get something that they can afford and you feel is fair. After all, a happy customer is a repeat customer.)
3) Do I want to make money now, or wait on it for a while and see if I can get a better deal? (This can be based on #2... if the person who wants to buy is simply well under your price range, there's nothing wrong with saying no and waiting to find someone else.)
To illustrate an example, I've sold tracks for well under what they were worth but still for an amount that was fair enough in the interest of solidifying a relationship with a label that could give me tons more work over the years. Prove you can do the job, are accountable, caring and flexible. You are providing a service, and you are fortunate enough that you can make unlimited product and don't necessarily have a "price" on creation. (Obviously you spent potentially years of time and money into your studio and craft. You know what I mean.)
Always keep in mind what we're really doing when trying to sell - providing a service to artists, labels, music supervisors etc. They need your music, and you want to show them that you can get the job done right and within budget.
It's always a careful balancing act. All I'm really trying to say is that there is a range of prices that my work commands given the purpose of the party interested in the music, the budget of the party, and the realistic potential of the track.
Hope this helps a bit!
