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Part 6 of 7 · Restricted fund tracker series ~5 min read

What the restricted fund tracker costs

The model reads invoices as they arrive, and grant agreements when they are signed or varied, which is a handful of documents a year. Fund balances, allocations and reports are arithmetic over records that already exist. Four hundred invoices a month is a charity with a busy building and several projects running at once. Here is where each cent goes.

Key takeaways

  • About $4 a month at 400 invoices. Roughly $10 at 1,200 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.
  • Grant agreements are the longest read per document and the rarest. Seven grants means perhaps a dozen agreements and variations a year.
  • 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 restricted fund tracker at three volumesA stacked bar chart with three bars, one per volume tier: 120 invoices totalling about $2, 400 invoices totalling about $4, and 1,200 invoices totalling about $10. 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$5$10$15$20~$2.09120 invoices~$4.03400 invoices~$9.581,200 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 400 invoicesHow it scales
Bedrock read$2.00Linear. One call per invoice, roughly 1,800 in and 200 out tokens.
SES$0.23Linear. About 0.8 messages per invoice.
DynamoDB + S3$0.50Storage 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 invoice, against a small, fast model. An invoice is a short document with a supplier, a date, an amount and a description, and the model’s job is to propose a fund and a category with a sentence of reasoning. Agreements are longer and are read by the same model a handful of times a year, which does not move the monthly figure. Eligibility is decided in code against approved rules, so nothing here needs 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 invoice when a variation arrives. A variation starts a new rules version from its effective date. Costs are re-tested against it in code; nothing needs reading by the model twice.
  • Sending the payroll export through the model. It has a fixed layout and a fixed set of columns. Parse it. A model over it costs money every month and, occasionally, moves a decimal point in somebody’s pay on its way into a funder report.
  • Keeping every scanned invoice in standard storage indefinitely. Grant agreements and the charity’s own record-keeping set retention periods that are long but finite. Tier documents once a grant is closed and expire them at the longest period any agreement requires.

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. 120 invoices, about $2. 400 invoices, about $4. 1,200 invoices, about $10. 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~$1120 invoices~$2400 invoices~$41,200 invoices~$10One 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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