Pay as you go AI: 9 routes, and $5 buys 500 flagship answers

What apps can you pay as you go for AI?

Nine routes let you pay for frontier AI per use, and only three are pay as you go from a zero balance with a chat window attached: Google AI Studio, OpenRouter and NanoGPT. Two more are developer consoles with a playground (OpenAI, Anthropic), one is a $39 licence on your own keys (TypingMind), and three are top ups bolted onto a subscription you must already hold (ChatGPT credits, Claude usage credits, Poe add-on points). We read every vendor's billing page on 27 September 2026; the table is what they say, in US dollars, at list prices.

Three rows are not what their marketing suggests. ChatGPT's credits, which OpenAI describes as "a pay-as-you-go add-on for supported features when you need more usage beyond plan limits", pay for Codex, ChatGPT Work and the Word, Excel and PowerPoint add-ins, and OpenAI's own FAQ says buying them "does not change your Free or Go plan or increase your included usage limit". Claude's usage credits bill at API rates but switch on only for Pro and Max subscribers, so the $20 comes first. Google is the exception in the other direction: its billing page says "AI Studio usage remains free of charge unless users link a paid API key for access to paid features", which makes AI Studio the one frontier chat window that costs nothing.

Microsoft 365 Copilot pay-as-you-go, which ranks for this query, is billed through an organization's Azure subscription and switched on by an admin, so it is not a route for one person with a card.

How much does pay as you go AI cost per answer?

A standard flagship answer costs between 0.7 and 1 cent at list prices: Claude Sonnet 5 is $0.007, Gemini 3.1 Pro Preview $0.008 and GPT-5.6 Sol $0.01 for 1,000 tokens in and 500 out, the unit the Whizi cost index prices every model on (OpenRouter list prices fetched 2026-08-20). Google's own pricing page carried the same $2 in and $12 out per million tokens for Gemini 3.1 Pro Preview on 24 September 2026.

The $5 minimum lasts longer than it sounds: 500 Sol answers, 625 Gemini 3.1 Pro answers or 714 Sonnet 5 answers, which at ten questions a day is seven to ten weeks. OpenRouter's fee changes that at the bottom end. Its 5.5% has an $0.80 floor, so the fee on a $5 purchase is 16%, and it only drops to the plain 5.5% once you buy about $15 at a time. Refunds are the other small print: OpenRouter allows them "within twenty-four (24) hours", Anthropic says "All credit purchases are non-refundable", and OpenAI refunds only for billing errors, unauthorized use or a confirmed service failure.

Two things push the real per-answer number around. A conversation resends its history on every turn, so the tenth message in a thread carries the nine before it, and ChatGPT API vs subscription works through why a ten turn thread costs about double the same ten questions asked fresh. In the other direction, the cheap rows are very cheap: GPT-5.6 Luna, the model OpenAI says runs Think mode on ChatGPT Go, is $0.0008 an answer, so $5 is 6,250 of them.

Where does pay as you go beat a $20 plan?

Prepaid credit beats ChatGPT Plus and Claude Pro, both $20 a month as of September 2026, until you send about 2,000 GPT-5.6 Sol answers or 2,857 Claude Sonnet 5 answers a month: 67 and 95 a day, every day, in fresh threads. In ten turn threads the Sol crossover arrives nearer 30 a day. Below those lines the top up wins on price, and most people asking this question are below them; above them the plan is cheaper, because 100 Sol answers a day is $30 of credit and Plus is still $20.

Against Whizi, which publishes this page, the per-answer arithmetic is worse for the subscription, and Whizi is a subscription with monthly credits, not pay as you go. Pro at $19.99 a month billed yearly ($29.99 monthly) carries 2,500 credits, and Claude Sonnet 5 costs 10 of them per answer, so the month is 250 Sonnet answers, which is $1.75 of API credit. Starter at $10.99 billed yearly ($15.99 monthly) is 800 credits across ChatGPT Luna, Claude Haiku and Whizi AI, with no flagship rows, and 800 Luna answers cost 64 cents on the API. Credits reset each month and do not roll over, where API credit lasts 12 months. The first week costs $0.99 on Starter, $1.99 on Pro and $4.99 on Powerhouse, then the plan renews unless you cancel; how credits work has the full scale.

What every plan sells instead of a per-answer price is the next section, and it is the whole decision.

What does a subscription buy that a top up does not?

A subscription buys the product around the model and a ceiling on the bill. A top up buys the model, at a floor with no ceiling unless you set one. ChatGPT Plus lists voice, image generation, file uploads, deep research and memory for its $20; the OpenAI Platform gives you an endpoint and a Playground, and every one of those features is a separate API, a separate bill and your own wiring. The same split holds at Anthropic and Google. A licence like TypingMind, or a free client, closes part of the gap on your own keys, and Poe vs TypingMind vs Whizi is the long read on what each closes.

The ceiling matters more than it sounds. OpenAI's prepaid billing page says it in one line: "Do not rely on the prepaid balance as an instantaneous spending cutoff." A runaway script, or a long thread over a pasted 300 page document, can spend a month of Plus in an afternoon. Auto reload is on by default when you set up OpenAI prepaid billing, so turn it off or cap the monthly limit the day you fund the account.

Then there is the clock. OpenAI, Anthropic and Google all expire prepaid credit 12 months after purchase, OpenRouter reserves the right to at 365 days, Poe add-on points last a year and its plan points "do not roll over, unless specified". A $5 top up you forget about is $5 gone in a year, which is still less than one month of any plan you forget to cancel. Is it worth paying for AI is the page for the person still deciding whether to pay at all.

Which pay as you go AI route fits which person?

One vendor, under ten questions a day: that vendor's own console. Google AI Studio costs nothing until you link a paid key, and $5 at OpenAI is 500 Sol answers with a Playground to type them into. Several models on one balance: OpenRouter, $5 plus the fee, with a chat room and 50 free requests a day on the free models. A real app on your own keys, with projects and file uploads: TypingMind at $39 once, then the keys. Under $5 to see whether you care: NanoGPT from $1. Already on Plus, Pro or Max and hitting the caps: the vendor's credits, which for ChatGPT means Codex and the Office add-ins rather than chat, and for Claude means API rates inside the app you already use.

A subscription, from any vendor including this one, is the right buy when the month runs past about 70 flagship answers a day in fresh threads, or 30 in long ones, or when the phone app, memory and file tools are the reason you open the thing. Below that, price the month before you pick: count a week of questions, multiply by four, then by a cent. Under $20, top up $5 and leave auto reload off. Over it, buy the plan and let the plan be the ceiling.