Word of mouth, blog tours, signing books already shelved at a book store, author readings, requesting reviews, interviews, and talking with book store purchasing managers and librarians cost nothing and constitute the bulk of effective marketing for your book.
Most authors will spend their own money on purchasing bookmarks and other promotional goodies for readers. This can cost anywhere from under $20 to over $500 depending on the number of items you purchase and the design cost. Other minimal costs for marketing include small advertisements on review sites that cost between $10 and $20 per month.
When you receive a contract from a reputable publisher, the contract comes with a marketing budget, which the publisher will pay for. The budget includes advertisements, supplying books for signings and advance reader copies to reviewers. Additionally, the publisher may pay an author's travel expenses for a book tour. Not every author is offered a book tour. If your contract does not cover a book tour, you can arrange one yourself at additional cost.