Black and white vs. colour
The use of colour on the book’s cover makes no difference to cost, but the use of colour inside a book may increase the cost considerably.
In many cases, colour should only be used inside if it is really warranted, e.g. colour pictures to illustrate the different rose varieties, to clarify complex diagrams, or to show vintage hotrods in all their splendour.
If you really need colour on the inside of your book, consider the inclusion of colour sections rather than spreading colour throughout. This will help keep the cost down.