Graphic designers talks a lot about that... and somehow, in that matter, our job is similar to theirs.
Here's some reference I've grabbed in the past few weeks :http://justcreatived...or-design-work/http://www.allfreela...and-businesses/http://freelanceswit...ing-your-price/http://freelanceswitch.com/rates/
Personally, I'm okay with Hans' prices. The last map I made took me about a day/a day and a half and I charged 450$ (Canadian $, roughly 325 euros). It was an historical map of the city of Quebec for a novel which include some research and retracing almost everything. I did provide with my quote an estimate of the time I think it would take (I have a rate for research and another for mapping), but in the end, it's the global amount I charge so I'm thinking of removing my hourly estimate...
I tend to charge more and more by the project and not at the hour because I'm faster now at my job than I was 4 years ago so why should I charge 30% less if I can make the map in 7 hours instead of 10 when it will also be of a better quality? You spend a lot of time working on your terrain, but the next time, it will be faster and charging by the hour, you will lose money (or regain a little, if you lose a lot this time
I've once read, even though I can't find the source, that to estimate your hourly cost (because you still need that to estimate the total cost of a project), multiply what you want to have in you pocket by at least 1.8. Under that ratio, you are "loosing" money, because I you say that you'd be happy with, let's say, 20$ an hour, you could lose in taxes 30-35% (at least here in Quebec) and you will have to pay what an employer would normally pay (software, insurances, pension, sick days...). You will lose maybe half of it. The consultant firm for which I work for my day job (as a GIS specialist) is "selling" me for 2.35 times my salary.
For my first job as a cartographer
(unashamed of putting the link... I still proud of that book), I charged more that half of what I'm charging now when a graphic designer was, him, charging the full price to put colors on my map... Now I do it all and charge it all
Also, but charging to low, you will give a sense of low quality to your work. My fiancee and I are currently looking for a wedding cartographer and when we finally found one charging half the price of the others, I started to think "why so low? Is he good or what?"