We measure our effectiveness by sales, but I've spent hundreds in the past 2-3 months advertising one particular book, targeting the readers of books and authors I think are a good match. Sales have barely covered the ad spend, never mind the royalty.
This book was released on Kindle in 2011. Over the past 5-8 years the also-boughts have been a mess, and I swear I've never seen it in the carousel for another title on Amazon (also-boughts, also looked at, recommended) unless I paid to show it on the product page.
Well, slowly, gradually, it's been creeping into view, and now when I look at random targets I see my book in their also-boughts and also-looked-at.
As a result, the daily sales have steadily increased until they're sitting nicely above the 90- and 30-day average. And 90% of these sales are in the US, which tells me it's the visibility and the amazon algos leading the charge, because I'm paying the same amount to advertise in the UK and sales there are currently minimal.
So what's my point?
You cannot measure your AMS ad performance on sales alone. There's an invisible hand at work, guiding the amazon algos so that more of the people looking at your book are likely to buy it.
How to make this work?
First, spend on ads targeting specific other titles related to your book. In my case, popular humorous scifi like Space Team, Earthcent, Bob and Nikki. This can be expensive, because people using a scattergun approach and paying heaps for bids will be all over the books you're targeting.
This step can take weeks. You need sales, even if they're costing more to get than you're earning.
Now, once those targeted books start showing in the Also-boughts on YOUR product page, you can fire up auto-ads. These should be cheaper than the targeted bids above, and because you've already trained AMS to a certain extent, they should show your book on the same product pages above, but at a lower cost.
As you make sales, keep checking several target product pages, going through the also-bought carousel until (hopefully) you see your book. Also check the ads carousel to see if your auto-ad is showing. If it's on page 40 or something, you'll need to increase your bid.
I started all this in Sept/Oct last year, and only now am I seeing the benefits. It's like all the various parts of the AMS and Amazon platform have finally meshed and I can see the steps to (hopefully) reproduce it.
I spent way too much in the earlier months, mostly on expensive auto-ad bids which I was running before Amazon knew who to show them to. Running the targeted campaigns first, training the algos, is the key.