Since 3 out of 4 threads in the employment discussion is a question about how much to charge for a job, I decided to make a sticky about pricing advices.
You pros are invited to contribute to it, that way we can point posters to it rather than repeat the same info 36 times a week.
My first tip is to get this book Graphic Artist's Guild Handbook Pricing & Ethical Guidelines
--------Product description from Amazon
The most respected single reference for buyers and sellers of creative work returns in an updated ninth edition, containing the latest, essential information on business, pricing, and ethical standards for nearly every discipline in the visual communications industry--from advertising to publishing to corporate markets.
My second tip is if you are not from North America, look into local artists associations an guilds they often have pricing guidelines, even if these are not as comprehensive as the GAG handbook.
My third tip is in case what you are doing is not listed in the book or you simply don't have the book. Evaluate the time the job will take you, add time for revisions, decide on a per hour rate, multiply it by the time you will need. Then evaluate the materials needed, add to the amount for time or quote separately. There you are. It's probably the best way to rate jobs like game assets or portrait commissions. Log the time you spend on project so that you can better evaluate the time you need later for similar assignements.
My fourth tip is not so much about the price per say. Make a quote template with your terms of services on it and a personalized letterhead. Stating the rights you give for that price and your payment terms upfront looks professional.
Fifth tip, especially if you are working for a non-start-up company, try to make your client make an offer, you could be surprised that he offers more than you wanted to ask. The worst that can happen is that you have to make a counter offer, if they think it's too high, they would have thought that even if you had made the quote first anyways.