As @SallyMason1 mentioned it really depends on your budget. Marketing/promoting your book also takes time and energy. While you want to attract new readers you also want your marketing efforts to count. To do this you need to have a clear idea of who your target audience is and how to position your book in front of them. There’s a couple of things many authors including myself have tried to attract new readers that won’t break the bank.
Free Book giveaways. If you have a large following this could work. Otherwise sites such as Goodreads which connects writers to readers is another great place to host a giveaway. Plus you increase your visibility by running a giveaway.
Contacting book reviewers/bloggers in your genre/niche. You want to get as many book reviewers lined up (if you haven’t already) who are interested in your genre. Having more reviews (especially positive ones) adds credibility and helps undecided readers make an informed decision on whether your book is worth taking a chance on. Plus if your reviewer really likes your book they will promote it for you via word of mouth. Word of mouth is free and can be very effective especially if that book blogger has a large and engaged following.
You can also experiment with some of the promotion tools within Amazon KDP which include:
Kindle Countdown deals
Amazon ads (this can be run against books of similar genres that happen to be getting traction)
I just scratched the surface. Ultimately you want to strive to build a platform and use a mix of paid marketing and self-promotion (social media, blogs, email newsletters, and etc.) to promote your book.