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Part 6 of 7 · Cost anomaly alerter series ~5 min read

What the cost anomaly alerter costs

A cost monitor that costs real money would be embarrassing, and this one does not: it makes a handful of API calls once a day. Thirty runs is one account watched daily. Here is where each cent goes.

Key takeaways

  • About $2 a month at 3 accounts. Roughly $5 at 10 accounts.
  • One Bedrock read per daily run is the only line that scales. Everything else is rounding.
  • Nothing is always-on, so a quiet month genuinely costs almost nothing.
  • The Cost Explorer API is charged per request, which is the only unusual line and is still pennies at one run a day.
  • The duplicate test runs before the read, so resends are free.
  • The three real risks: a retry loop, storage nobody expires, and a bigger model than the job needs.

The bill at three volumes

These are US East prices at the time of writing, at three volumes that bracket most small businesses. Find the bar closest to your own and read across.

Monthly cost of the cost anomaly alerter at three volumesA stacked bar chart with three bars, one per volume tier: 30 runs totalling about $2, 3 accounts totalling about $2, and 10 accounts totalling about $5. Each bar is stacked from bands. The largest and fastest-growing is Bedrock, one read per daily run, in teal. Then SES for the messages, in pink. Then S3 and DynamoDB storage in green. Then a fixed orange band for Secrets Manager and AWS Budgets, which is eighty-six cents at every volume. Then a grey band for Lambda, SQS and CloudWatch. A note says the read is the only bar that grows with the business and the orange band never moves.$0$2$4$6$8~$1.5930 runs~$2.283 accounts~$4.710 accountsBedrock — one read per daily runSES — asks, results, receiptsCost Explorer API requestsS3 + DynamoDBFixed — Secrets Manager, BudgetsLambda, SQS, CloudWatchThe read is the only bar that grows with the business. The orange fixed band never moves.
Fig 1. The monthly bill at three volumes. The teal band — one model read per daily run — is the only part that grows; the orange fixed band is the same 86 cents at every volume.

Line by line

LineAt 3 accountsHow it scales
Bedrock read$0.00Linear. One call per daily run, roughly 1,800 in and 200 out tokens.
Cost Explorer API requests$0.90Linear at $0.0100 per daily run.
SES$0.05Linear. About 0.4 messages per daily run.
DynamoDB + S3$0.29Storage grows with what you retain, not with throughput.
Lambda + SQS$0.12Linear, and effectively free at this scale.
CloudWatch$0.16Flat, if you set retention. Unbounded if you do not.
Secrets Manager$0.40Flat. One secret, $0.40 a month.
AWS Budgets$0.46Flat. Two actions, so you find out before the bill does.

The Cost Explorer API is one of the few AWS APIs charged per request. At one grouped query per day per account it is a few pence a month; at one query per service per day it is not.

The three ways this bill surprises you

Every one of these has happened to somebody, and all three are cheap to prevent.

  • Querying Cost Explorer per service per day. The API charges per request, and a loop that asks separately for each of thirty services on each of three days is ninety requests a day rather than three. Group the query.
  • Hourly cost queries. Tempting for faster detection and the wrong solution: the data does not update hourly, so it is ninety-six times the API cost for no additional signal. Use usage metrics for speed instead.
  • Log retention left at never. With a bill this small, unbounded logs will be the majority of it within months.

What it costs when nothing happens

This matters more than the headline number for a seasonal business. In a month with nothing to process the bill is the fixed band: Secrets Manager at forty cents, AWS Budgets at forty-six, and a few cents of storage. Call it a dollar. There is no instance to stop and nothing to remember to turn off.

The monthly bill at four volumes plus one failure modeA horizontal row of five boxes. Quiet month, about one dollar. 30 runs, about $2. 3 accounts, about $2. 10 accounts, about $5. And one bad retry loop, about two hundred dollars. A note says four of these are the design working and the fifth is a missing dead-letter queue.THE BILL, AT A GLANCEQuiet month~$130 runs~$23 accounts~$210 accounts~$5One bad loop~$200Four of these are the design working. The fifth is a missing dead-letter queue.
Fig 2. The bill at a glance, including the one that is not a volume at all. A retry loop with no dead-letter queue costs more than every legitimate use of the system put together.
  • Management
  • Analytics
  • Front-end & mobile

Set these on day one

  • A dead-letter queue on every SQS queue, with a maximum receive count of three.
  • Thirty-day retention on every CloudWatch log group. There is no default that is safe.
  • An S3 lifecycle rule on the object prefix, tiering at 90 days and expiring at your actual record-keeping horizon.
  • Two AWS Budgets actions — one that emails at half your expected spend, one at double it. The second is how you find out about a loop in an hour instead of a month.
  • Provisioned concurrency: none. Nothing here is latency-sensitive enough to justify paying for a warm function.

Next: the same system drawn for engineers — service names, resource identifiers, IAM scopes, table schemas and the model id.

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