[Groblek]

I set my rates for commissions based off the Science Fiction Writers of America's minimum for "professional rates" for short stories. At the time I looked, that was $0.05/word or $50 for 1,000 words. I did that for two reasons - first, I've been writing and posting unbirth stories for about fifteen years and figured I have the name recognition to get away with it. Second, I don't have a lot of free time, so the time I do have is quite valuable to me. At that rate, I get very few commissions, and they tend to be short, which is pretty much what I'd hoped for. Three 3,000 word scenes are a lot easier for me to write than a single 9,000 word story.

So, I'd say the rate depends on your goal. If you feel low on ideas or enjoy writing to others' specifications, set a low rate and you'll get more commissions. If you want to weed out all but the really serious people, set a higher rate. If your main goal is to make money off your writing, I'd suggest polishing your work enough to self-publish through Amazon. It's a lot easier to get someone to pay $2.99 for an ebook of a story type they like than it is to get them to pay $40+ for a custom story.
Cheers,
Groblek