$25 a Month, With a $16 AI Stop Line

01 / The $25 receipt before the first request

My rule for a side project is simple: it must survive a quiet month without becoming a financial obligation. For this example, the entire monthly budget is $25. That is a planning limit, not a claim about what every application costs.

Here is the receipt I would approve before shipping a small AI-powered utility:

• ■ Application hosting: $5 maximum.
• ■ Database: $0, using an existing allowance with paid overages disabled.
• ■ Monitoring: $0, using basic logs and free checks.
• ■ AI API usage: $16 maximum.
• ■ Unallocated buffer: $4.
• ■ Total authorized spending: $25.

These are budget allocations, not vendor quotes. If hosting actually costs $7, something else must lose $2 before launch. Include taxes, currency conversion, storage, and any other billable extras in the calculation. A spreadsheet that excludes them is not a hard cap.

02 / The $16 AI line needs a gate, not an email

A notification at $15 is useful, but it does not stop a background worker, a retry loop, or five simultaneous requests. The application needs an admission check before every billable attempt.

CheapRelay fits the variable-cost part of this setup: it has no monthly fee or subscription, and billing follows real tokens, including cache reads and writes. That removes a fixed subscription line from this receipt; it does not automatically guarantee a spending ceiling.

For the worked example, assign each permitted request a conservative maximum cost of $0.02. This is an illustrative engineering envelope, not a CheapRelay price quote. Your actual envelope must come from current rates and enforced request limits.

• ■ 700 production attempts × $0.02 reserved = $14.
• ■ 100 development and test attempts × $0.02 reserved = $2.
• ■ Total admission budget: 800 attempts and $16.

Retries count as attempts. So do scheduled jobs, evaluations, and requests made while debugging. If a request cannot fit inside its reservation, reject it or shrink it before sending it.

03 / A two-cent reservation must cover the whole request

Calculate the worst-case bill using the chosen model's current input, output, cache-read, and cache-write rates. Bound the input size and output tokens. Include any other applicable billable features, or disable them. Recheck the calculation whenever pricing or configuration changes.

If those limits produce a maximum above $0.02, the example's 800-attempt allowance is invalid. Raise the reservation and reduce the attempt count. Never round a worst-case cost down to make the budget look comfortable.

For a deliberately conservative first version, keep the full reservation charged against the local monthly allowance even when the actual bill is lower. For example, 800 requests averaging $0.013 would cost $10.40, while the admission ledger would still show $16 committed. That unused headroom buys simplicity; it is not a promised average price.

04 / One atomic counter prevents the $16 race

Store the remaining allowance in a shared database, not separately inside each application process. Atomically reserve $0.02 before dispatch. If the balance is insufficient, send no request. If the database is unavailable, fail closed.

Keep the API key server-side and route every caller through that gate. A forgotten script using the same key bypasses the entire design. Keep timed-out attempts charged: a client timeout does not prove the upstream request was free. Disable automatic retries unless they also obtain reservations.

05 / The $4 buffer stays off-limits to ordinary traffic

When the AI allowance runs out, show a plain message: monthly AI capacity is exhausted. Offer non-AI functionality or wait until the next budget period. Do not silently switch to another paid service.

Reconcile actual charges before resetting the ledger, and define the monthly boundary explicitly. The $4 buffer covers small billing surprises; it is not permission to keep generating.

Sign up for CheapRelay if pay-as-you-go access suits this budget, then put your own spending gate in front of it. The useful feature is not unlimited access. It is knowing that a popular weekend cannot authorize a larger bill.