Depends on the subject matter you're writing about, experience, and country the person is from that's paying. If you're looking on a site such as odesk, the BIGGEST thing you can do is make sure people pay a % upfront on fixed fee's. There's a ton of scammers out there that offer jobs. Let you do 20 hours of work, then *poof* magically take off with out ever paying. I'd say 3 dollars for 500 words is reasonable though. Usually it's better to set yourself up as a virtual assistant and work for an hourly rate to make sure you actually get paid than to offer freelance writing gigs.