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Part 6 of 7 · Document expiry watcher series ~3 min read

What the expiry watcher costs

The watcher is one of the cheapest systems in this whole series. The daily tick reads a CSV from S3, does some date arithmetic, writes a few rows to DynamoDB, and posts a handful of messages to Slack. It calls no models on the tick. Bedrock fires only when somebody forwards a contract PDF and once a month for the board summary. At typical SMB volume, the bill is a couple of dollars a month, fixed cost essentially zero.

Key takeaways

  • Around $1.50/month at typical SMB volume (around 100 tracked items).
  • Fixed AWS cost is essentially zero. No always-on compute, no NAT Gateway, no API Gateway.
  • The daily tick costs pennies — no model calls.
  • Bedrock fires only on inbound PDF parsing (a few times a month) and the monthly summary.
  • At 500 tracked items the bill is around $4. At 2,000 items it’s around $12.

Cost at three volumes

Monthly cost at three tracked-item volumes, broken out by component A horizontal stacked-bar chart showing monthly cost in US dollars at three tracked-item volumes. The leftmost bar represents 100 tracked items and shows a total around $1.50, dominated by the everything-else slice (Lambda, DynamoDB, S3, EventBridge Scheduler, CloudWatch) with a tiny sliver for Bedrock and a tiny sliver for Textract. The middle bar represents 500 tracked items and shows a total around $4, with the same shape — the everything-else slice grows roughly linearly with item count because the daily tick still reads every item. The rightmost bar represents 2,000 tracked items and shows a total around $12, with everything-else still dominant; Bedrock and Textract stay small in absolute terms because they only fire on the inbound parsing lane (a handful of forwarded contracts per month) and the monthly summary. Below the chart is a legend explaining the four sections of each bar: Bedrock (only on inbound parsing and monthly summary), Textract (only on PDFs forwarded to the inbox), AWS Budgets and Secrets Manager (small fixed amounts), and an everything-else bucket for Lambda runtime, DynamoDB on-demand, S3, EventBridge Scheduler, SES (inbound and outbound), and CloudWatch. A note at the bottom: the daily tick is the dominant cost, and even that is fractions of a cent per item per day. $0 $5 $10 $15 $20 100 items ~$1.50 500 items ~$4 2,000 items ~$12 Bedrock (inbound parsing + monthly summary) Textract (forwarded PDFs only) AWS Budgets + Secrets Manager (fixed) Everything else (Lambda, DDB, S3, Scheduler, SES, CloudWatch) The daily tick is the dominant cost — and even that is fractions of a cent per item per day.
Fig 6. Monthly cost at three tracked-item volumes. Bedrock and Textract are small slivers because they only fire on the inbound parsing lane and the monthly summary. The dominant cost is the everything-else bucket: the daily tick reading every item.

Where the dollars actually go

Lambda runtime (the bulk). The watcher runs once a day. Each tick reads the registry CSV from S3, iterates the rows, computes days_to_expiry for each, and decides on a move. At 100 items, that’s a few hundred milliseconds. At 2,000 items it’s a couple of seconds. Either way it’s pennies a month. Add the dispatch Lambda firing for each ping (around two to ten pings a month at 100 items, twenty to fifty at 2,000), the Function URL Lambda for acknowledgments, the calendar-sync Lambda running hourly, and the drive-sync Lambda every fifteen minutes — the Lambda total still lands under a dollar at all three volumes.

DynamoDB on-demand. Three small tables: ew-pings, ew-ack, ew-audit. Reads are dominant during the daily tick (one read per item per tick, plus chain history). Writes are dispatch events and audit rows. Pennies a month at any of these volumes.

S3 + Storage. The mirrored registry CSV plus the archived MIME from any forwarded contracts. A few hundred KB total at SMB volume. Effectively free.

EventBridge Scheduler. The daily tick rule plus deferred dispatch rules from quiet-hours and holiday gates. A few invocations a day. Pennies.

SES. Inbound for the forwarding lane: $0.10 per thousand received messages (so a couple of cents a year for an SMB). Outbound for email-fallback alerts: $0.10 per thousand sent. Both are negligible at this scale.

Bedrock (only when something fires it). The daily tick uses no Bedrock. The inbound parsing lane fires Haiku 4.5 once per forwarded PDF: a few thousand input tokens (the Textract output) and a few hundred output tokens (the proposed row JSON), so a fraction of a cent per parse. At a few forwarded contracts a month, Bedrock costs cents. The monthly summary is one larger call: write a paragraph that summarizes the month’s pings, renewals, and lapses; a couple of cents.

Textract (only on forwarded PDFs). Per-page pricing; a typical contract is two to ten pages. A few cents per parse. At a few PDFs a month, Textract is a few cents to a dollar. At 2,000 tracked items with twenty PDFs forwarded per month, it lands around a couple of dollars.

What doesn’t cost money

  • API Gateway. Replaced by Lambda Function URLs for the acknowledgment endpoints.
  • NAT Gateway. Nothing is in a VPC. No NAT, no $32/month minimum.
  • Always-on compute. No EC2, no Fargate. The watcher sleeps 23.99 hours a day.
  • A Knowledge Base. The registry is structured rows, not free text — deterministic lookup beats vector search here. No embeddings, no Knowledge Base, no S3 Vectors needed.
  • Models on the tick. The daily decision is plain Python. Bedrock fires only on the inbound parsing lane and the monthly summary.

How the cost scales

Lambda runtime grows roughly linearly with item count, because every item is evaluated on every tick. DynamoDB grows linearly too. Bedrock and Textract are uncorrelated with item count — they only fire when somebody forwards a PDF or it’s the first of the month. So the bill at 5,000 tracked items is around $30; at 10,000 it’s around $60. Past those volumes the daily-tick model probably stops being right (you’d switch to a partial-tick that only evaluates items inside the union of all category windows), but those are optimizations for very large catalogs — not redesigns.

Set an AWS Budgets alarm at $15/month so anything unusual pages you before the bill matters. The watcher’s normal-volume bill stays well under that ceiling.

Last post in the series: the engineering reference. Same system, drawn for engineers — service names, Lambda inventory, IAM scopes, DynamoDB schemas, SES rule set, and EventBridge Scheduler config.

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