Footnotes are added to a book at the typesetting stage. They make the job of typesetting much more complex and slow, and this will be reflected in the price if you’re paying someone to do it. You should ensure that any footnotes or captions that need to be added to the book are supplied to the typesetter in a Word document from which they can copy and paste the text. Never allow them to type anything themselves unless you’re standing over their shoulders watching closely.