Why every engine campaign has a $100 daily budget
Each engine campaign carries a $100 daily budget (or the local-currency equivalent). It is a ceiling that keeps learning unblocked, not a spend target — your ACoS or TACoS cap governs actual spend. How to set a hard limit if you need one.
TL;DR: Each campaign the engine creates carries a $100 daily budget, or the local-currency equivalent. That is a ceiling so a campaign is never starved while it learns; actual spend is governed by the ACoS or TACoS cap on the product, and stays far below the budget. For a hard limit on total daily spend, use Amazon's account-level daily budget cap.
If you open your Amazon Ads console you will see every campaign the engine created with the same daily budget: $100, or the equivalent in your marketplace's currency. Sellers notice, and reasonably ask whether that is what they are about to spend.
A ceiling, not a target
The engine builds many small campaigns — typically one per keyword or target — and optimises each one's bids toward the product's goal. The budget's only job is to make sure a campaign is never cut off mid-day while it is learning; a campaign that hits its budget at noon stops collecting the data the engine needs. So every campaign gets the same generous ceiling, and real spend is controlled somewhere else.
What actually governs spend is the target on the product: the engine bids to keep your actual ACoS or TACoS under the cap you set, and lowers bids when a campaign is not earning its keep. A single-keyword campaign at a sensible bid spends a few dollars a day, not a hundred. Across the account, spend follows your caps, not the sum of the budgets.
If you need a hard limit
Your target keeps spend efficient; it does not promise a maximum. If you need one — a cash constraint, a launch you want capped — set an account-level daily budget cap in Amazon Ads (Sponsored ads settings → Daily budget cap). Amazon then stops serving ads for the day once total spend reaches your number, across every campaign, whoever created it. The engine works normally underneath it.
Changing individual campaign budgets by hand is not the tool for this: the engine manages those campaigns, and lowering one budget does not bound the others.