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Part 6 of 7 · Holiday accrual tracker series ~5 min read

What the holiday accrual tracker costs

The model reads contracts, and a business hires far less often than it runs payroll. Everything that happens per shift, per period and per booking is arithmetic against a table. Here is where each cent goes.

Key takeaways

  • About $3 a month at 120 workers. Roughly $7 at 400 workers.
  • One Bedrock read per worker is the only line that scales. Everything else is rounding.
  • Nothing is always-on, so a quiet month genuinely costs almost nothing.
  • The bill tracks headcount, not hours. Doubling the rota does not move it.
  • 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 holiday accrual tracker at three volumesA stacked bar chart with three bars, one per volume tier: 25 workers totalling about $2, 120 workers totalling about $3, and 400 workers totalling about $7. Each bar is stacked from bands. The largest and fastest-growing is Bedrock, one read per worker, 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$2.5$5$7.5$10~$1.6125 workers~$2.95120 workers~$6.88400 workersBedrock — one read per workerSES — asks, results, receiptsS3 + 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 worker — is the only part that grows; the orange fixed band is the same 86 cents at every volume.

Line by line

LineAt 120 workersHow it scales
Bedrock read$1.44Linear. One call per worker, roughly 1,800 in and 200 out tokens.
SES$0.10Linear. About 3 messages per worker.
DynamoDB + S3$0.35Storage 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.

Roughly one model call per employment contract, against a mid-tier model. A contract is long but the extraction is narrow: the leave year, the entitlement, the rate, and whether there is a written term about recovering overtaken leave. That is four fields and a quotation for each, which is not a job that improves with a frontier model.

The three ways this bill surprises you

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

  • Re-reading every contract on every payroll run. A contract changes when somebody signs a new one. Extract once, store the terms, and re-read on a version change.
  • Recomputing the whole history every night. The ledger is append-only precisely so that a balance is a sum over new lines, not a replay of four years.
  • Keeping every timesheet import forever at full fidelity. Keep the derived ledger line, which is small and permanent, and expire the raw import once it has been reconciled.

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. 25 workers, about $2. 120 workers, about $3. 400 workers, about $7. 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~$125 workers~$2120 workers~$3400 workers~$7One 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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