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Part 6 of 7 · CIS deduction checker series ~5 min read

What the CIS deduction checker costs

The model reads subcontractor invoices, one call each, and a contractor with fourteen subcontractors sees around forty of those a month. Everything else is subtraction, a rate lookup and a return built once a month. A hundred and fifty invoices a month is a contractor running several sites at once. Here is where each cent goes.

Key takeaways

  • About $3 a month at 150 invoices. Roughly $6 at 500 invoices.
  • One Bedrock read per invoice is the only line that scales. Everything else is rounding.
  • Nothing is always-on, so a quiet month genuinely costs almost nothing.
  • Receipts attached to an invoice go into the same read, so collecting the evidence does not add a second call.
  • 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 CIS deduction checker at three volumesA stacked bar chart with three bars, one per volume tier: 40 invoices totalling about $2, 150 invoices totalling about $3, and 500 invoices totalling about $6. Each bar is stacked from bands. The largest and fastest-growing is Bedrock, one read per invoice, 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~$1.6340 invoices~$2.75150 invoices~$6.28500 invoicesBedrock — one read per invoiceSES — 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 invoice — is the only part that grows; the orange fixed band is the same 86 cents at every volume.

Line by line

LineAt 150 invoicesHow it scales
Bedrock read$1.20Linear. One call per invoice, roughly 1,800 in and 200 out tokens.
SES$0.09Linear. About 1.2 messages per invoice.
DynamoDB + S3$0.37Storage 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 subcontractor invoice, against a mid-tier model, with any attached receipts in the same call. An invoice here is a page or two of lines to be classified as labour, materials, plant, consumables, fuel or travel, plus a VAT line and a few words of reverse-charge wording. That is classification with awkward formatting, and a frontier model reads the same lines for several times the price.

The three ways this bill surprises you

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

  • Reading every merchant receipt separately. A groundworker’s month can arrive with thirty receipts. They go into the invoice’s single read as attachments and only their totals come back; a call per receipt multiplies the bill by the size of the paper pile.
  • Re-reading invoices at the monthly close. The close works from the stored read. Building a return or a statement should never touch the model, and a close that re-reads to be sure pays twice for every invoice in the month.
  • Keeping every attachment in standard storage indefinitely. The CIS minimum is three years after the tax year ends and most contractors keep longer. Tier attachments once the month closes and expire them at the horizon you actually chose.

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. 40 invoices, about $2. 150 invoices, about $3. 500 invoices, about $6. 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~$140 invoices~$2150 invoices~$3500 invoices~$6One 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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