The cover is sometimes what sells the book. This comment is made with no offense intended to the writer and their intense contribution to the content within those two covers. If you are lucky enough to be one of the familiar authors who hit the best seller's lists - then you are more than likely to recognized and purchased for your name and style of writing or genre.
An author has to remember, as well, that cover art is usually not a choice, unless you happen to be the artist as well as the writer, or you happen to be one of those best selling authors with a lot of creative influence. In any case, I have heard that contributions of your own artwork is permitted in the publishing industry. The publisher, however, with the market and promotional background and experience to help you sell your book, might advise otherwise. They may work with you to suggest something that might be more appropriate or better suited to the book market today.
Flashy vs. understated? Depends upon who you are trying to attract and what image you want to portray. Color vs. black and white? Again, it really depends upon the content and your audience. Kid's books are definitely better in full, bright colors.
Artwork for the children's selections above were a combination of black and white and color illustrations. Black ink outlines were used to increase the images and shading with artist's pencils - black lead or color - were used for highlights, depth, and shadow. The artwork was done on artist's sketch paper in a large sized format that was scanned and emailed for the publishing lay-out process. The books are available in print or download from the sources indicated above.
The cost for illustrations and cover work varies sometimes as much as the style of artist. Every artist has a unique quality that distinguishes their work from others - some are able to draw as if photographs and the accompanying price will reflect their masterful work. Others draw with more animation, possibly with less realism, but are equally qualified to supply images for a book project. Just be specific what you are looking for and be diligent in your search to find the right artist. It will be expected that the author promote the artistic contributions with a by-line of the illustrator's name. Very often artists will use their published work for portfolios and recognition of the writer should be reciprocated by the artist. It makes good business sense to enter into a contract when commissioning the work of an artist for your book - outline the requirements, expectations, and deadlines. It doesn't sound like "creativity" but it will protect both parties, in case something comes to dispute.
Wildhorse Creative Arts provides all types of artwork from logo design to cover design, so if you are looking for a quote for a special project, please use the "contact us" form. If you are a writer, artist or a photographer looking for a book publisher that will work with you on all aspects of the publishing process, including artwork, if required - email firstname.lastname@example.org