Let me understand. You intend to have a site where the value of the service will be $500?? Really?
Have you done a huge research if this have some possibility of success?? I'm not saying that it can't, I'm just asking. Because other sites like Freelancer, 0desk, and Elance are dealing with values around this one, so, what can you offer to make your site very attractive?
You have to understand that the economy in the world is not strong yet and that's why that the Fiverr appeared, it saw a big niche to be exploited.
Before you start this site you better make a big research to assess the real possibilities.