The referral fee is a percentage of the price
Amazon charges a referral fee on most categories as a percentage of the sale price, so the fee moves every time the price does. Enter the rate for your category and the page multiplies it against the price you typed: at $24.99 with the common 15%, the referral fee is $3.75.
Rates differ by category and Amazon revises the schedule, which is why the default is a starting point rather than a claim about your listing. The field accepts up to 45%, enough for the categories that charge most, and using the wrong category taxes the answer systematically instead of by a rounding error.
Because the fee scales with the price, a lower price also lowers this line. That does not make discounting free, but it does mean the referral fee is not a fixed cost per unit the way fulfillment is.
The FBA fee is a flat dollar per unit
Fulfillment by Amazon bundles pick and pack, weight handling and outbound shipping into a single charge per unit, set by the product's size tier and weight. On this page it is one dollar figure you supply, and it does not change when the price does.
That flatness matters most on cheap products: the same $5.20 fee takes about 21% of a $24.99 sale and nearly 35% of a $14.99 one. The same fee stack can therefore make one product profitable and another unprofitable without anything about the selling change.
It is an estimate rather than a quote, because size tier, weight and program membership all move it. Keeping it as an input is deliberate: a fetched number would go stale quietly, while a number you type is one you can check against your own fee preview.
Profit is the price minus all three removals
The page subtracts the product cost, the referral fee, the fulfillment fee and any other-cost allowance from the price and prints each line, so the total can be argued with. At the defaults, $24.99 minus $8.00 of product, $3.75 of referral and $5.20 of fulfillment leaves $8.04, which is a 32.2% margin on the sale price.
It also solves the same equation backwards: the price at which a unit just covers product cost and the fixed fees at your referral rate. At 15% that is $8.00 plus $5.20 divided by 0.85, or $15.53 with no other costs. Price below that line and every additional sale deepens the loss.
One detail is worth knowing before reading the statistics: the "Amazon keeps" figure adds the referral fee, the fulfillment fee and the other-cost allowance together, so it also carries any packaging or returns allowance you entered rather than only what the marketplace takes.
What the margin does and does not include
The margin on the sale price is the honest headline for comparing one product with another, because a percentage normalizes for price while dollars are what pay the bills. A 20% margin on a $50 product beats 30% on a $10 one by a wide margin in dollars, and the page shows both figures.
It stops at the per-unit line on purpose. Monthly storage, returns processing, sponsored products and your own time are all real costs, and each varies with volume and season, so folding them into a single figure would be a guess dressed up as arithmetic. The other-costs field exists if you want a deliberate allowance included.
A selling price of zero stops the page instead of returning a number, and zero units sold clear zero dollars. The page prices one unit at the fees you entered, which is a different question from how many units the market will buy.
A per-unit figure only pays bills by the month
Clear profit is quoted per unit, but storage, ads and app fees are billed per month, so the two only meet through a division. At the defaults a unit clears $8.04; $1,500 a month of storage, advertising and tools needs 1,500 divided by 8.04, or 187 units, before the product has covered anything beyond itself.
That division is where a healthy per-unit number stops being reassuring. At $14.99 the same fee stack leaves -$0.46, and no number of units divides a loss into a profit - volume only makes it larger. Cutting the price to win that volume usually raises the break-even count instead of lowering it, which is why the page prints the unit profit and the margin side by side rather than the percentage alone.
Danfio