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Part 6 of 7 · Pension enrolment assessor series ~5 min read

What the pension enrolment assessor costs

The model reads what staff write, and nothing else. Assessing every worker in every pay run is a comparison between a number and a threshold row, and it costs rounding error whether the payroll holds twenty people or two thousand. Ninety messages a month is an employer in an enrolment or re-enrolment month. Here is where each cent goes.

Key takeaways

  • About $2 a month at 90 messages. Roughly $4 at 400 messages.
  • One Bedrock read per message 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 messages, not headcount. Assessing five hundred people every week costs cents more than assessing fifty.
  • 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 pension enrolment assessor at three volumesA stacked bar chart with three bars, one per volume tier: 20 messages totalling about $1, 90 messages totalling about $2, and 400 messages totalling about $4. Each bar is stacked from bands. The largest and fastest-growing is Bedrock, one read per message, 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~$1.420 messages~$1.990 messages~$4.12400 messagesBedrock — one read per messageSES — 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 message — is the only part that grows; the orange fixed band is the same 86 cents at every volume.

Line by line

LineAt 90 messagesHow it scales
Bedrock read$0.45Linear. One call per message, roughly 1,800 in and 200 out tokens.
SES$0.06Linear. About 2 messages per message.
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 message, against a small, fast model. A message about the pension is two or three sentences, and the job is to pick one or more of four kinds and check for a signature or a personal-submission statement. That is classification with a short extraction. A larger model buys nothing here except a stronger temptation to let it write the reply, which it must never do.

The three ways this bill surprises you

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

  • Sending the payroll export to the model. It is a fixed-header file with a row per worker per pay period. Parse it; a model asked to categorise workers would be slower, dearer and occasionally wrong about a threshold by 75 pence.
  • Re-reading a whole thread every time a worker replies. Each message is read once, on arrival. A worker’s history is a query against stored classifications, not a prompt with the conversation pasted into it.
  • Keeping every raw payroll export forever. The assessments, the duties and their evidence must be kept. The weekly export behind them does not need to sit in standard storage once it has been assessed and reconciled to the pay run.

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. 20 messages, about $1. 90 messages, about $2. 400 messages, about $4. 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~$120 messages~$190 messages~$2400 messages~$4One 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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