BRIEF
An AI API key costs nothing to create, so here is what you actually pay for
The key itself is free. You pay only for what you use, from prepaid credit that starts at about $5. For a small business drafting a few hundred emails a month, that usually comes to somewhere between 35 cents and a few dollars, depending mostly on which model you pick.
Connery Davoodian · Founder, Fynl ·
The key is free, and you pay per word from prepaid credit
Creating an API key with Anthropic (Claude) or OpenAI (ChatGPT) costs nothing. The key is a password that lets software use the AI on your account. You are billed for what that software actually sends and receives, measured in tokens.
A token is a chunk of text. OpenAI's own rule of thumb is that one token is about four characters of English, or roughly three-quarters of a word. So 1,000 tokens is about 750 words. Every request has two parts that are priced differently:
- Input tokens cover what you send: the customer's email, your instructions, any notes about your prices or policies.
- Output tokens cover what the AI writes back. They cost several times more per token than input.
Here is how billing works at both companies. You sign up at the developer console (console.anthropic.com or platform.openai.com), add a card, and buy credit up front. When we last checked, the minimum top-up at both was $5. Usage is drawn down from that balance. If the balance hits zero, requests stop working until you top up again, unless you have turned on auto-reload.
That means there is no monthly fee and no per-seat charge. A month where nobody uses it costs $0.
What the rates look like
Rates are quoted in dollars per million tokens. A million tokens is roughly 750,000 words, about eight novels. The table shows list prices Anthropic and OpenAI published in late 2025. Model names and prices change, so check the pricing pages linked below before you budget.
| Model | Input, per 1M tokens | Output, per 1M tokens | Good for |
|---|---|---|---|
| GPT-5 mini (OpenAI) | $0.25 | $2.00 | Routine replies, sorting, summaries |
| Claude Haiku 4.5 (Anthropic) | $1.00 | $5.00 | Fast everyday drafting |
| GPT-5 (OpenAI) | $1.25 | $10.00 | Harder writing, quotes, longer documents |
| Claude Sonnet 4.5 (Anthropic) | $3.00 | $15.00 | Careful writing, contracts, nuanced replies |
| Claude Opus 4.1 (Anthropic) | $15.00 | $75.00 | Rarely needed for small-business admin |
The gap between the cheapest and most expensive row is the number that matters. Opus costs 60 times more per input token than GPT-5 mini. For drafting a reply to a wedding inquiry or a brake-job quote request, you will usually not see a 60-times better answer.
A worked month: 300 drafted email replies
Take a photographer, or a body shop, that gets about ten inquiries a day and has the AI draft a reply to each one. That is 300 replies a month. A realistic single request looks like this:
- Input: about 1,500 tokens. That covers the customer's email, a paragraph of instructions, and your price list and availability notes.
- Output: about 400 tokens. That is a reply of roughly 300 words.
The arithmetic for Claude Sonnet 4.5 works like this. Input is 1,500 × $3 ÷ 1,000,000 = $0.0045. Output is 400 × $15 ÷ 1,000,000 = $0.006. Together that is about 1 cent per reply, or $3.15 for the month. Run the same numbers on each model and you get:
- GPT-5 mini: about $0.0012 per reply, $0.35/month
- Claude Haiku 4.5: about $0.0035 per reply, $1.05/month
- GPT-5: about $0.0059 per reply, $1.76/month
- Claude Sonnet 4.5: about $0.0105 per reply, $3.15/month
- Claude Opus 4.1: about $0.0525 per reply, $15.75/month
To get your own number, open the usage page in the console after one normal week. It shows tokens and dollars per day. Multiply the week by four and a bit. That is more reliable than any estimate, including this one. For what other jobs cost, such as call summaries, quote drafts, and review replies, see what AI can actually do for a small business, and what each job costs.
What people get wrong about the cost
A ChatGPT Plus or Claude Pro subscription does not include API use. The $20-a-month chat subscription and the API are billed separately, even under the same login. If a tool asks for an API key, your Plus plan will not cover it. You need credit in the developer console.
Usage is rarely the expensive part. Choosing the wrong model is. In the example above, doubling your email volume on Sonnet adds about $3. Switching from GPT-5 mini to Opus multiplies the whole bill by about 45. Start on a cheap model and move up only for the jobs where you can see the drafts are worse.
A runaway automation is the real risk, not normal use. An automation stuck in a loop can send thousands of requests in an hour. Set a monthly spend limit in the console, which both providers offer. Leave auto-reload off until you know your normal monthly figure.
Anyone selling a free or cheap API key is selling a stolen or shared one. Keys are tied to the account that pays for them. Make your own.
Should you get one at all? If you only want to chat with an AI yourself, a $20 ChatGPT Plus or Claude Pro subscription is simpler and the better fit. A key makes sense when software is doing the work for you: drafting replies, filling in quotes, summarizing inquiries. In that case you want to pay the provider's rate, not someone's markup on it. Fynl's AI runs on your own key for this reason, so the provider bill is your whole AI cost. Many other tools bundle AI into a per-seat price instead, which is worth checking before you compare plans.
Questions people also ask
Is there a monthly fee for an OpenAI or Claude API key?
No. Creating the key is free and there is no subscription. You buy prepaid credit (the minimum was $5 at both when we last checked), and each request is deducted from that balance based on the tokens it uses.
Does my ChatGPT Plus subscription include API access?
No. ChatGPT Plus and the OpenAI API are billed separately, and the same is true of Claude Pro and the Anthropic API. You need to add credit in the developer console to use a key.
What happens when my API credit runs out?
Requests fail until you top up, so any tool using the key stops producing AI drafts. You can turn on auto-reload, but set a monthly spend limit first so a broken automation cannot drain your card.
How do I keep my API key from being misused?
Treat it like a password. Only paste it into tools you trust, set a spend limit in the console, and delete the key and make a new one if you think it has leaked. Deleting a key stops it working immediately.
Sources
One portal instead of five subscriptions
CRM, calendar, quotes, invoices, website and email — with the AI running on your own Claude or ChatGPT API key, so there is no markup on it.
See pricingMore posts