More background: I'm renegotiating my first contract ever as a primarily php developer and am not sure where to estimate market rate.
To quick and dirty it:
Low end: Check out Rentacoder.com.
Mid Range: $15-$23/hour contract or ~$40,000-$45,000/year salaried outside the valley or NYC, $65,000-$80,000 inside the valley or NYC.
Higher End: $24-$50/hour contract ($75-$100 on short term, specialized, mission critical gigs) or ~$45,000-$70,000/year salaried outside, $80,000 - $120,000 inside.
Guru++: A lot. Also, be sure you have a well stocked fridge of beers from many nations, a ping pong table and hipster art on your wall. :-)
I want to stress that the number of conditions that go into the pay number defy database columns. Ask for a number a little higher than you think and see what they say...
One way to also see a skill may be by job title in your region from job sites like careerbilder or monster though many jobs don't give salary and the ones that do may give you an untruthful range, but it is something.
You're not a barrel of oil. Your value is highest to someone who is developing a very valuable project who has a need for your skillset.
Where you live has a huge impact on this. I'd speculate that you're more likely to find a higher value being in a place like Silicon Valley. Note also that the value of the stock options you earn depends enormously on how well you choose a project and team with potential.