In general there are two different ways to estimate price for illustration and design work: hourly and flat rate. It's important to establish which type of estimate you are using, because the range for each can be very different.
Hourly quotes can seem lower than flat rate quotes, but it's important to remember that the final cost may vary by a great deal with an hourly quote. Even well planned and properly estimated freelance projects can go over budget. Your price may therefore increase a substantial amount. For example, a freelance illustration estimated to take 30 hours might take twice as long. If you are paying by the hour, you might find that your cost is much higher than you expect.
It's important to know which kind of quote you're dealing with. Having a firm budget in mind up front can really help guide you as to which type of quote will work best for your business.