Like what Julie said, they're worth it if you plan to do other marketing activities as well. (post on socials, growing your email list, getting on podcasts, etc.) Amazon Ads makes sure you show up in more places, but it doesn't guarantee sales. So if you're doing promotions, AND your book shows up on people's Amazon searches multiple times, that's what'll drive more sales. One note though - with Amazon Ads, you get to see what people are searching for which is valuable data as well.