How much should you charge? As much as you possibly can. Seriously! For several reasons:
- you want the client to value you and your craft
- you want the client to value audio as well
- you want to stand out from the rest. Make great content and don't be afraid to charge for it
- you want to keep the audio profession alive and thriving.
- you want to make this your business. So aim as high as you can!
Try to avoid:
- working for peanuts (or even worse, for free!)
- being vastly undercut by counter offers. In other words, don't be afraid to say no
Set a number that makes you feel good about yourself and your work. A number that will make you feel good about the transaction. Nobody likes working for super cheap. Don't fall into some of the common traps some devs will try and throw at you (i.e. "I'll pay you on the NEXT game!" "This will be a HUGE break for you!" "We'll do profit sharing and we'll all get rich") Be picky about who you work with and what you are work.
Figure out how much it costs for you to pay rent/mortgage, pay your bills, have a little bit of fun, eat food and live your life. Then figure out how much time it takes for you to finish a track. And I don't mean when you, the composer, feels done but rather when your client feels supremely happy with your work.
Here's a big one: you mention often how much money you've spent on your DAW/samples. That's only part of it. You've also spent your time learning, practicing and studying music/production/etc. Consider that as well.
One follow up:
If you set a rate and then after 6-8 months you've had zero sales, something's off. Either in your rate, your actual music or the way that you're marketing/promoting/branding yourself. I never freak out when a dry spell of a month to three months comes along. They always do and then it passes. Remember the saying "feast or famine" applies heavily to freelancing. But after 6-8 months, you SHOULD have at least one sale/commission/gig for your work. If not, then it's time to reassess and try new tactics, write/produce different music or adjust your rates.
Best of luck!