I would not recommend to invest money in external marketing, as long as you have not exhausted all of the free possibilities.
It starts with the possibilities inside of Amazon: It's absolutely clear that you need a perfect cover, perfect keywords, perfect description, high quality content and so on. But have you ever thought about the perfect pen name? Every niche has a specific audience, with particular preferences. Do they want to read a book by a male or female author? What age do they prefer? Should the name sounds short and hard, dynamic, traditional or maybe soft? How would the person look like? (> choose the perfect photo for your pen name). What could be the perfect story behind your pen name? And at the end you should write your book(s) from the perspective of your pen name! (Providing good information is not enough because everyone can do this). Finally you should give your target audience exactly what they want even if they're not sure that they want it. They want it subconsciously. That's psychology, that's marketing, that's your primary job!
(and not to buy Facebook). So you should invest your money into knowledge about marketing, psychology, how to analyse your target audience, how to manipulate people with your writing style and so on. Success is just based on the right descisions and you need logical reasons for every step of your steps. So if you can't give yourself the answer why you're using this font on your cover, this pen name, this sentence in your biography etc. you should learn about it. Otherwise your success is pure luck.
