Translating your book into other languages opens up international markets for you. The market for Spanish literature, for example, is very large. Also consider languages like French, German and Russian. The cost of translating a book varies, depending on the type of text. Translating a straightforward non-fiction text in a genre like Self Help is generally inexpensive and can be quickly done.
For a more complex work like a literary novel or an academic textbook however, you will require a highly skilled translator who can convey the subject matter and nuances across languages accurately. Such translators are specialists who will charge higher fees and require more time for the translation process.