I just charge my the type of music and the type of media that the music or sound effect go in.
But one thing I keep in mind all the time is the idea of networking and building client relationship. Sure, you may charge $5,000 for a 7 minute score with a deadline of 2 days, but will the client come back to you if they are not as desperate? I doubt it. So when I give out a quote, I want to make sure that the client and myself are comfortable with it enough that they have the possibility of coming back to me later in the future for more business.
And I simply charge by individual tracks if it’s for pure music production. But when it comes to motion picture scoring, I charge based on the budget of the client. If an entire film budget is $100,000 USD, I’d probably charge like, ionno, $20,000 USD IF the film requires a full live orchestral composition. However, if it’s a pure simple and electronic score like the movie, “The Social Network,” I’d probably charge less than that, maybe $10,000 USD? I dunno, it really varies on what the client offers.
Usually the client gives me a range of what they are able to offer, and I find a price that is not too demanding nor too low for my time 
Cheers,
Thomas