Back when I was freelancing the single best piece of advice I got was to keep on raising my rates until no one would hire me. It's a very simple way of finding out the price the market will bear for your skills.
Well, that way you will end up working on ridiculous things with stupid or noob clients. In the end, that kills your value. Customers who want real stuff also know how to hire so they won't hire overpriced devs.
Actually I ended up working on fantastic projects for some excellent clients, but I had a strong personal reputation and a great network backing me up as well.