Here's a great article that discusses how much we should charge for our work. It is written by Emmett Cooke.
It goes through the thought process of how much it costs us, and how much it costs the client.
After that it goes to discuss a few ways we can calculate our costs, and different ways we can charge the clients.
I believe it not only applies to just music composing, but can be really useful for any industry where you do a task-based project.
I got to know Emmett Cooke from this website he set up, its called VstBuzz.com. A music composer for media, games, TV and films, he is an active member in the music community. VstBuzz.com is his way of bringing great virtual instruments to the community at promotional/discounted prices. Do check it out!
After every purchase from VstBuzz, Emmett never fails to write in to see if everything is fine with the purchase and if the product is working well.
I applaud his passion for helping out the community, and I really love his music.