build.allanninal.dev

Part 6 of 7 · AWS autoposting series ~3 min read

What this all costs

A coffee a month, not a Netflix subscription. Here’s where the dollars actually go.

The bill stays small because the system sleeps when there’s nothing to do. There’s no always-on server idling between events. Every cost line is either zero or proportional to actual use.

A quick note about AWS’s free tier

AWS changed how the free tier works in July 2025. New accounts now get six months of access plus up to $200 in credits, instead of the old twelve-month version. The always-free pieces this article relies on weren’t touched — the small list below is still free for everyone, old accounts and new. The math here works either way.

Cost breakdown by tier: free, fixed, variable, and total Three stacked panels showing AWS costs by tier. Always free at this scale: Lambda, EventBridge, DynamoDB, SQS, SNS, and Lambda Function URLs. Small fixed costs: Secrets Manager about one dollar twenty cents per month, S3 storage plus Vectors index ten to fifty cents per month. Variable with use: Bedrock Titan embeddings at about two-hundredths of a cent each, Bedrock Haiku replies at about seven-hundredths of a cent each, with one thousand replies per month working out to about seventy cents. Total realistic monthly cost: two to five dollars per page. A budget alarm at ten dollars catches anything weird. Always free at this scale $0.00 / month Lambda 1M requests + 400K GB-s/month forever EventBridge Scheduler 14M invocations/month DynamoDB 25 GB storage always free; on-demand req < $0.01/mo here SQS · SNS · Lambda Function URLs covered by the free tier at this volume Small fixed ~$1.30–$1.70 / month Secrets Manager (3 secrets: FB token, Drive key, app secret) ~$1.20 S3 storage + S3 Vectors index ~$0.10–$0.50 Per Facebook page. Adds linearly with each new client page. Variable with engagement ~$0.60–$2.30 / month Titan Text Embeddings v2 ~$0.00002 / 1K tokens Claude Haiku 4.5 replies ~$0.00125 each Claude Haiku 4.5 guardrail judge ~5% of posts Worked example: 1,000 replies in a month ≈ about $1.25 of AI cost. Realistic total $2 – $5 / month per Facebook page, with steady posting and replies AWS Budget alarm set at $10/month catches anything weird before it grows.
Fig 6. Three tiers: free, small fixed, variable with use. Plus a hard ceiling alarm at $10.

The brain got cheaper to store last December

Until late 2025, the standard place to keep the brain (the searchable copy of the client’s docs) was a service called OpenSearch — which costs real money even when nobody’s using it. In December 2025, AWS launched a much cheaper option called S3 Vectors. For one Facebook page, the brain is small (well under a gigabyte) and gets a few hundred lookups a day. On the new service, that’s pennies a month. If you’d planned this a year ago, you can take a whole line off the bill.

The traps you’re deliberately avoiding

Most cloud bills go sideways because of a few specific decisions made early. The architecture above sidesteps each of them on purpose:

  • No always-on networking gateway. The networking gateway most cloud setups need (called a NAT Gateway) is about $33 a month before you send a single byte through it. The robots here don’t need it — they talk to other AWS services on the internal network and to Facebook over the regular public internet. So the line item is zero.
  • No API gateway. The robots receive webhooks directly. The fancy version of the gateway is $3.50 per million requests; the cheaper version is $1 per million. We use neither. (If you do end up needing one later, pick the cheaper version.)
  • No bottomless log bucket. Every log stream is set to delete itself after seven days. Logs can’t quietly pile up and get billed for years.
  • No keep-the-engine-warm fee. It’s fine for the robots to take a moment to wake up cold — we’re not running a low-latency website. Paying to keep them warm would be money for nothing.
  • No CDN, no private network endpoints, no container registry. Nice things to have for big systems; not needed for this one.

The math that makes this work

The trick isn’t cleverness — it’s discipline about what doesn’t exist. Every always-on piece was swapped for one that wakes only when there’s work to do. Every paid service was checked against a free alternative first. The result is a steady bill made of three small line items: a few cents for storage, a dollar or two for the password vault, and the AI calls you actually made that month.

Adding a second Facebook page roughly doubles only the variable bucket and the storage line. The free-tier components stay free. So the marginal cost of a new client page is closer to $1–$3/month than the $30+ a traditional setup would cost.

A coffee a month

For one page with steady posting and replies, all-in cost is about $2–$5/month. For five pages, more like $10–$20/month. The architecture scales with the variable line, not with a fixed-cost staircase.

All posts