One of the most common questions that people ask when faced with the cost of custom artwork or illustration is, "why is art so expensive?" It's a good question, and it deserves a good answer. While it's easy to assume that art is fun, therefore artists must not mind making art even if they don't get paid much for it, that assumption doesn't apply to professional artists and illustrators. Aside from the very simple fact that art ceases to be fun when you're undervalued and overworked, keep in mind that you are paying a professional for their time, effort, and expertise.
If you hire me to do custom illustrations for your book/magazine/museum exhibit/educational materials etc., you aren't just paying me for the cost of materials or the time it takes me to complete the illustrations. You're paying me for the years I've put into learning and honing the technical skills that scientific illustration requires; for the work I put into obtaining my bachelor's degree in biology, my master's in zoology, and my certificates in scientific illustration and technical writing; and for the decades of hands-on experience I have working with animals and studying specimens. You're paying me for my advanced technical skills and extensive knowledge base.
Another thing to be aware of if you are commissioning illustration work for a commercial or public project (as opposed to commissioning a painting private enjoyment) is that you are also paying for reproduction and usage rights. The more extensive usages and rights a client wants, the more the illustration is worth.
A final important point to make about freelance illustration is that this is my business; I am self-employed and I not only have to pay myself fairly, I also have to pay business taxes and self-employment taxes. Art is fun! Accounting is less fun.
If you do choose to work with me, once we have discussed the scope and specifications of the project, I will send you an itemized proposal/estimate and contract establishing agreed-upon fees and rights. Once this is finalized to both parties' satisfaction, I will send you an invoice. For estimates and invoices I use FreshBooks software, which uses the WePay secure payment gateway. Alternative payment methods can be discussed, if necessary.