A monthly AI subscription buys access under a plan's rules. An API price measures programmatic consumption. Both can involve the same model, but dividing a subscription fee by an API rate does not reveal a guaranteed number of messages.
Identify the billing unit
Read the number and the words immediately beside it. Is the price per person, per workspace, per million tokens, per generated second, or per credit package? These units are not interchangeable.
- A subscription may include a changing allowance for selected models and tools.
- An API request may involve input, output, cached input, and tool charges.
- A credit balance is an intermediate unit. Its value depends on what a particular action consumes.
- A bundled API credit is a specific benefit with its own terms; do not assume every chat subscription includes one.
The official OpenAI API rate card distinguishes token categories and other billable resources. The official ChatGPT and Codex guide separately describes subscription usage. Keep those two sources separate in your calculation.
Use a transparent example
Suppose a fictional API charges $2 per million input tokens and $8 per million output tokens. A request with 10,000 input tokens and 2,000 output tokens would cost $0.02 plus $0.016, or $0.036, before any other charges. These are invented rates for arithmetic, not the current price of any listed model.
If your application sends a long document again on every request, your input total changes. If caching applies, use the provider's documented cache rules. If an agent calls tools or retries, include those operations in the estimate. Estimate a complete task, not just a single ideal response.
Annual prices need a second look
A page displaying $12 per month with annual billing implies $144 for a full year if that rate applies to all twelve months. It does not promise that you can cancel after one month having paid only $12. Record the actual amount due, renewal amount, taxes, and refund terms before you decide.
Build a comparison you can maintain
For each candidate, write down the plan name, currency, billing interval, included allowance, overage rule, official source, and date checked. Leave a missing value unknown. A login screen or a failed fetch does not mean the product is free.
Start with a small representative workload and review the provider's usage dashboard. For coding work, count completed changes you can accept. For creative work, count usable outputs after revisions. Cost per useful result is the number that connects the plan to your own needs.