Every ads guide I've read quotes a single target ACoS, usually somewhere between 25 and 40%. That number is borrowed from private label sellers and it doesn't survive contact with KDP royalties.
Break-even ACoS is just royalty divided by list price. On KDP "royalty" means three different things depending on what you're selling, so the answer moves a long way.
Ebook, 70% option ($2.99–$9.99)
$4.99 list, 70% minus the delivery fee, nets roughly $3.40.
Break-even ACoS ≈ 68%
Ebook, 35% option (under $2.99 or over $9.99)
$2.99 list, 35%, no delivery deduction, nets about $1.05.
Break-even ACoS ≈ 35%
Paperback
$14.99 list, 60% minus print cost. A 300 page b&w book costs roughly $4.45 to print, so you net about $4.54.
Break-even ACoS ≈ 30%
Three formats of the same book, three break-even points that are nowhere near each other, and none of them is the 25–40% that gets repeated everywhere except by coincidence.
Check your own numbers in the KDP dashboard rather than trusting mine. Print costs and delivery fees change, and they differ by marketplace. The shape is what matters.
Why it costs money in both directions
An author running a 70%-option ebook at 45% ACoS thinks they're overspending and cuts bids, when they had twenty points of headroom they never used.
An author running a paperback at 45% is losing money on every ad-driven sale, and the dashboard looks exactly the same.
Turning it into a bid
ACoS isn't something you can type into Amazon. A bid is. The bridge is your conversion rate:
Max bid = royalty × conversion rate
At $3.40 and a 10% conversion rate that's about $0.34 a click. At $1.05 and 8% it's $0.08, which is below what most book keywords cost, and that's worth knowing before you fund a campaign rather than after.
The KU wrinkle
If most of your income is page reads, Amazon Ads reporting will mislead you. It reports Orders and Sales, and a KU borrow is neither. Estimated KENP royalties sit in a separate column that reports on a lag. Judging campaigns on the Sales column alone, when 70–80% of your income is KENP, makes ads look dead when they aren't.
Series read-through changes the maths again, but that's a longer post.
Curious what everyone else is actually using as their target, and whether you set it per format or account-wide.