There appears to be very little information available about competitive hourly rates for GIS freelancers. Additionally, GIS freelancers never post rates on their websites. Elance has some information on what people are asking, although I believe the rates are well below the industry standard due to the structure of their bidding system (example). The US Bureau of Labor Statistics states the median pay for Cartographers and Photogrammetrists is $27.62 USD, although this is presumably low because it does not taking into account fringe benefits. What is the going rate for GIS freelancers? I am interested in hearing from both freelancers and businesses who contract out work.
Firstly - this is an excellent question (+1). As a freelancer, I wish I had had this information when I started out!
Secondly - YIKES! Translating that rate into sterling is frightening. GIS professionals must be ten a penny in the US is all I can say if that it the limit of your earning expectations.
Third (and a more serious commentary expanding on Conor's answer) - The answer to your question depends a lot on the sector in which you are applying your GIS skills. This is first and foremost the deciding factor. If I was working in the oil industry I would be charging double what I am at the moment (but GIS for the oil industry is a little bit specialized and some of the skills very niche e.g. bespoke geological software skills required rather than just ArcGIS or what have you).
I set my rate initially by taking my (then) annual salary (gross of tax to allow for some corporate tax and sundries as very roughly equivalent to my income tax and UK National Insurnce) and divided by 200 to give me a day rate. I used 200 because I used to work in a commercial GIS group where there was a concept of '200 chargeable days' - the remaining 165 days being taken up by weekends, sick leave, holidays, personal development, business development (including website maintenance), computer maintenance etc. This was a formula which worked well in my experience. For an hourly rate I divided the day rate by 7 because:
Using this simple calculation you can work out what you personally must charge to maintain your current standard of living. Anything less would represent an unacceptable drop in salary and mean it is not worth going freelance.
The aim of going freelance is to get flexibility, self determination, personal satisfaction and because you have noticed how your current employer is benefiting from your skills and you would like a cut of that money. The above calculation is just a baseline and to improve on it, you can put your prices up and you work harder once you get a feel for the market. However, this method of calculating your hourly rate will reflect the market you personally work in and the level at which you are working (as Conor says, it depends on whether you are a junior technician or doing advanced spatial analysis for instance).
ADDITIONAL COMMENT: I should add that, after all the talk about a minimum acceptable rate, it isn't just about the money. As a freelancer I personally have gained a lot more job satisfaction and can meet my clients' needs without all the bureaucracy of a large organisation getting in the way. I am also a lot more hands-on, which I enjoy and which I was losing with increasing seniority in a traditional employed role. This sense of satisfaction and achievement are worth a lot and can't be expressed in a simple monetary rate. This is the hidden bonus (though I grant you, you can't eat it and it doesn't pay the mortgage).
It really depends on what kind of work you are doing and the client you are doing it for.
What you do matters:
A freelance GIS Developer like Kirk Kuykendall is going to make a lot more per hour as a GIS Developer with a very specialized skillset than someone who is just digitizing a few thousand points or lines (technician level work) in a GIS.
Who you are working for matters:
Private industry generally pays more than local governments. Local governments generally pay more than nonprofits. Larger clients generally pay more than smaller to medium sized clients.
That being said, here is what I see for small to medium-sized clients:
For GIS developers (anecdotally based on work that I've done) the going rate isn't much different from most other freelance programming jobs ($70-125 USD per hour in the USA, depending on client, type, and length of work.)
I've seen technician jobs as low as $15 per hour (but more often closer to the $27 you quoted) and as such jobs falling more under the "Analyst" paradigm are somewhere in between.
This old web link from 2001 says that they charged $40/hour for their GIS Analyst work. Though the face of GIS has changed significantly since then, I don't find that to be an unreasonable number.
An hourly pay for a cartographer is of course not equivalent to what the same person with the same skill level would charge as a freelancer.
@Conor sugested $70-125 USD. That sounds about right on the low to medium end even here in Canada but could be more for highly specialized tasks such as specialized analysis and development work.
This does not mean however that you will take home $70-$125 USD / hour.
Subtract the cost of doing business business:
Add it all up and initially you may get the $30 - $50 out of the $70-$125 you will bill out.