I assume from your choice of forum you're talking synth stuff, not going out and recording for real? If so, I can't think of what forum to use either.
I'm afraid I can't give a very helpful answer other than "as much as the market will bear". Since the time it will take will vary hugely with the complexity of what they're asking for, maybe work out a fair hourly rate (don't forget to include the fact you're using expensive gear, not just you!) and give them an estimate of how long it will take you.