There are some important principals to understand before the business practices make sense. What you do as a professional artist is not create art, but sell copyright. All your artistic skills are used so you can create copyrighted material that someone else wants to buy. The value of this product has only a little to do with the amount of effort you put into it. You’re not hammering nails, this is intellectual property.
The main thing that affects the price is usage. So if you spent 5 hours or 100 hours on a painting, the main factor on the price is how many people will see it. The same illustration will sell for $200.00 for a pamphlet or $2,000.00 for a paperback cover.
The other factor is how big is your client. When you are dealing with commercial organizations, nothing is ever free. You don’t say “Oh it was only a sales pitch, or it was good for…