Norman Rockwell used often Watercolors on paper for studies and sketchess.
They are also are valuable auction pieces, fetching $1000 - $ 3000 (e.g. 2009 prices) and more.
You can try to find images and prices such as this belwo with links provided at the bottom of the page.
Being an illustrator, seven days a week, I haven't found anywhere he used watercolors as for, say Winslow Homer, that is fresh and luminous artistic final works. Rather it was a quick technique to lay down first ideas and glimpes for him, over pencil sketches I would presume. A way to give color.
Tom Sawyer, illustration,1935. One of the 8 oil paintings illustrations used to illustrate a deluxe edition commissioned him. of Tom Sawyer and Huck Finn which the Heritage Press
The same picture but as a preliminary sketch in pencil, ink and watercolor (valued $40'000 -$60'000). .You can see a full picture here (invaluable.com)
For a reason I don't understand, pictures, (just the pictures, not the real thing) of an artist work become copyrighted by others and they claim nobody cannot even show them. In the USA, this has been ruled out....
You can have an idea of the values of the "pictures" (no, I mean, the real thing) in these websites: