Cost Explorer tells you what you have spent when you ask. A budget tells you without being asked: it watches the month's spend and sends an alert when it passes a threshold.

A budget only alerts. Nothing is stopped or deleted; you read the email and decide.

Two files

The budget itself, in budget.json: a name, a limit, a period and what it counts.

{
  "BudgetName": "market-monthly",
  "BudgetLimit": { "Amount": "5", "Unit": "USD" },
  "TimeUnit": "MONTHLY",
  "BudgetType": "COST"
}

The alert, in alert.json: when it fires and who hears about it.

[{
  "Notification": { "NotificationType": "ACTUAL", "ComparisonOperator": "GREATER_THAN", "Threshold": 80, "ThresholdType": "PERCENTAGE" },
  "Subscribers": [{ "SubscriptionType": "EMAIL", "Address": "ops@monstermarket.example" }]
}]

ACTUAL means money already spent, and 80 with PERCENTAGE means 80 per cent of the limit: 4 USD.

Creating it

aws budgets create-budget --account-id 123456789012 --budget file://budget.json --notifications-with-subscribers file://alert.json

The Budgets API asks for the account on every call, so --account-id comes first: the twelve digits that aws sts get-caller-identity prints.

aws budgets describe-budget --account-id 123456789012 --budget-name market-monthly

This reads the budget back, with the spend so far under CalculatedSpend. The alert is read separately:

aws budgets describe-notifications-for-budget --account-id 123456789012 --budget-name market-monthly