What is your time worth? How much money do you need to keep food on your table and pay your rent? What is your pricetag?
Ideally, these are questions we illustrators need to be able to answer. If you don't have any of the answers, I suggest putting some thought into the matter. Some folks will arrive at specific figures for specific types of jobs. Others, will have a range of numbers in mind. These prices don't necessarily have to be rigid. Indeed, they can be quite situational, but at the very least you need to have a mental starting point for calculating a price when asked.
So how do you go about knowing what's worth what? One of my first stops has always been the Graphic Artists Guild Handbook: Pricing and Ethical Guidelines (a book I highly recommend owning if you are in the field or want to hire someone from the field). Sometimes you'll hear this referred to as the "GAG book" or the "PEGs." While not a complete list of the variety of jobs out there and their respective average fees, it is an excellent place to start. At the very least, you're libel to find a job that's similar to what you're doing and can extrapolate a fair price for your specific project.
Be warned, however, it has been my experience that the prices listed inside this book are a bit higher than I typically get paid. Such is life. The GAG book is not a rulebook. It states this fact in its title. It is a series of guidelines. So, don't be surprised should the negotiations trend lower.
The second thing I look at when coming up with a price is my own experience. After illustrating full-time for ten years, I have some familiarity with the rates in the illustration genres I've worked in. I have a pretty good idea what a given job in these fields pay. Once again, there are times when some extrapolation is necessary, but the numbers from the past can help inform the numbers of the future.
"But what if I don't have that kind of experience?" you inevitably ask. That's where the third thing comes into play: your fellow illustrators. Chances are that if you are an illustrator, you know other illustrators. At least one of these other illustrators will be willing to share information on prices. It's been my experience that it'll be more than one. Most of the illustrators I know will tell you the straight dope on what a given job paid them (depressing as it sometimes is). Your fellow illustrators are a valuable resource. Tap that resource.
Another way in which your network of illustrator friends and acquaintances is useful is when you're offered a job outside of your typical genre. Oftentimes different parts of the illustration world have vastly different fees and knowing illustrators who work in those different parts of the field can be invaluable. At the very least, it's possible that someone you know can refer you to someone else who can help you out.
Anyway, after taking those three things into account, you can start to settle on a ballpark figure. But we're not done. You see, there are a few more things to ask yourself. First off, how demanding is the job being offered? Does it require you to go well above and beyond your normal call of duty? What are the deadlines? How are these deadlines going to affect your personal life? For that matter, what is your schedule to begin with? Do you have the time for the job? The answers to these questions (and others you'll invariably come up with on your own) will undoubtedly have an affect on what you feel is a fair price. Consider these factors carefully.
I would hope that after pondering all of the questions and the information you've collected that you can arrive at a price. How firm you want to be on that price is up to you. It depends on what you're willing to live with, and how much of a hit you're willing to take.
Before I go, I want to mention one more thing: whatever price you quote, don't undersell yourself. While there are many out there who may overcharge, I think there are just as many (if not more) who will accept less than is fair. While there may be a variety of factors that lead to this (lack of self-esteem, financial necessity, desperation), I beg you to tread carefully. Though it is important to be realistic about your prices (for example, I can't command the fees that Michael Whelan can), don't settle for less than you're worth, either.