Skip to content
allanninal.dev/build
Collaborate

Part 6 of 7 · Petty cash tracker series ~5 min read

What the petty cash tracker costs

Petty cash volume is genuinely small — a busy office tin sees perhaps forty receipts a month, and four tins across a business is a few hundred. Nothing is always-on and the messaging line is almost nothing, because a matching count is silent. Here is where each cent goes.

Key takeaways

  • About $2 a month at 120 receipts. Roughly $3 at 400 receipts.
  • One Bedrock read per receipt is the only line that scales. Everything else is rounding.
  • Nothing is always-on, so a quiet month genuinely costs almost nothing.
  • Storage grows with retained photographs rather than with throughput, and a lifecycle rule keeps it flat.
  • 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 petty cash tracker at three volumesA stacked bar chart with three bars, one per volume tier: 40 receipts totalling about $1, 120 receipts totalling about $2, and 400 receipts totalling about $3. Each bar is stacked from bands. The largest and fastest-growing is Bedrock, one read per receipt, 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~$1.4640 receipts~$1.91120 receipts~$3.49400 receiptsBedrock — one read per receiptSES — asks, results, receiptsTextract — every receipt photoS3 + 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 receipt — is the only part that grows; the orange fixed band is the same 86 cents at every volume.

Line by line

LineAt 120 receiptsHow it scales
Bedrock read$0.31Linear. One call per receipt, roughly 1,800 in and 200 out tokens.
Textract$0.18Linear at $0.0015 per receipt.
SES$0.06Linear. About 0.3 messages per receipt.
DynamoDB + S3$0.30Storage 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.

Textract and the model together come to well under a cent per receipt, which is the entire variable cost of the system. Everything else on the bill is the fixed band.

The three ways this bill surprises you

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

  • Re-reading on every retry. A photo that fails once and is redelivered should not pay for Textract and a model call again. Key the read cache on the image digest so a retry is free.
  • Full-resolution photographs kept forever. Phone cameras produce several megabytes per receipt, and a few hundred a month for seven years is real storage. Store a downscaled legible copy alongside the original and expire the original at your record-keeping horizon.
  • Log retention left at never. At this volume the logs will out-cost every other line within a year. Thirty days of retention is the whole fix.

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 receipts, about $1. 120 receipts, about $2. 400 receipts, about $3. 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 receipts~$1120 receipts~$2400 receipts~$3One 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.

All posts