If you read up on AMS help, you’ll find that there are two things that drive ads – bid price and relevance. The higher the bid price on a keyword, the better the placement you get on the carousel. If you bid 20 and someone else bids 30, then all things being equal, that 30 bid ad will appear before yours. If enough ads appear ahead of yours when they win their slots, then your ad will appear on the fifth page or worse and will be rarely seen or clicked.
Things get tricky when relevance is added to the equation. If you have a book on healing and a keyword ‘murder’, it’s very likely that nobody will use ‘murder’ to find your book on ‘healing’. As an ad goes forth into the world, Amazon gives each keyword an average relevance score which is why ads do strange things at the start. As a keyword gathers impressions but no clicks, its relevance score goes down and that keyword ad slows down or stops because Amazon gets no money from it. This is why you want to prune weak performers and reward good performers. In this scenario, your keyword bid could be 40 but since it has a bad relevance score, another keyword with higher relevancy at a bid of 20 will appear above it.
There’s also another factor with AMS ads (sponsored). Amazon chooses a subset of your 1000 keywords to try out. I have no clue what drives the choices, only that sometimes it can fixate on a single keyword and suck up all your budget on that single one. I’ve found that tweaking down the bid price tends to jolt it into trying other keywords. But only do that if the keyword seems to not be performing well.
General keywords like ‘thriller’ or ‘mystery books’ are going to have higher bids because they are too generic. You want to try finding more specific keywords with less competition if you want to use bids under 50c. Try adding book titles and author names as keywords. Add which thriller tropes you are using as keywords. Try to think of other more specific searches people might type in Amazon to find a book like yours and add those. An example in romance would be ‘billionaire enemies to lovers’.
Which brings me to the final subject of pruning your keywords. Treat keywords like a Bonsai tree. You want to discourage under-performing keywords and reward good ones with bid nudges down or up. Let them run for two weeks and then examine the results. Prune if necessary. Wait another two weeks and rinse and repeat.