What we look at before spending anything
Two checks, and they're free to do. Both of them tell you whether paid promotion is even worth starting.
Does the page convert
Your retail page is the sales page. Cover at thumbnail size, the first three lines of the description before the fold, the look inside. If that page isn't converting the traffic it already gets, more traffic just costs more.
Is the category winnable
Some categories are crowded by books with thousands of reviews and a decade of history. Others aren't. Picking a category where your book can realistically rank is worth more than any campaign, and it's a decision, not a budget.