This price list is for the benefit of self-publishing authors who want to get an idea of the cost of illustrating a picture book.
The price includes storyboard, dummy, 1 rough sketch, and 1 final illustration. Pretty good deal, when you break it down! Again, that price is based on being assigned about 10 - 30 illustrations.

Here are the definitions of the different sizes:
Spread - an illustration that spreads across two facing pages, filling up both pages with a single illustration. It may or may not have text superimposed on it. 

Full - an illustration that takes up one whole page. It may or may not have text superimposed on it.

Half - an illustration that takes up one half of a page.

Quarter - an illustration that takes up one quarter of a page.

Spot - a simple illustration that takes up one eighth of a page, or less.