New models · Measured
What 100 Image Prompts Cost on GPT-6 Luna
GPT-6 Luna costs $0.10 per million input tokens and $0.50 per million output tokens — half of GPT-5.6 Luna's promotional price — since its release on September 22, 2026. Prices per million tokens are hard to picture, so we priced a real job.
On September 23 we asked Luna for 100 image prompts for one fictional bakery post and measured what came back: length, rule-following, variety and the likely cost at API prices. The cost turned out to be the least interesting number.
The request
We used ChatGPT Work with GPT-6 Luna at High effort. The job: 100 prompts for one 4:5 Instagram photo of a Pumpkin Rye Sourdough from Kettle & Crumb, a fictional neighbourhood bakery, varied in angle, setting, light, props and mood, each 20 to 35 words and each stating that the image has no lettering.
Write 100 different image prompts for one 4:5 Instagram photo of Pumpkin Rye Sourdough from Kettle & Crumb, a neighbourhood bakery. Vary the angle, setting, light, props and mood. Each prompt is one sentence of 20 to 35 words and must say that the image contains no words, letters or logos. Number them 1 to 100. Output only the list.What came back
Luna returned exactly 100 prompts in under two minutes: 2,928 words and 17,425 characters. Every prompt was between 26 and 33 words, so none broke the length rule, and all 100 carried the no-text clause.
The weakness was variety. Every prompt opened with one of four phrases: 'Create a 4:5…' 34 times, 'Photograph…' 33, 'Show…' 17 and 'Capture…' 16. Wooden surfaces appeared in 20 prompts and counters in 18; steam appeared once and hands twice.
| Check | Result |
|---|---|
| Prompts returned | 100 of 100 |
| Words per prompt | 26 to 33 (rule: 20 to 35) |
| No-text clause present | 100 of 100 |
| Different three-word openings | 4 |
| Most repeated prop | wooden surface, 20 prompts |
| Time | under 111 seconds |
What the text costs
ChatGPT shows no token counts, so the cost below is an estimate. English text runs at roughly four characters per token, which puts the list at about 4,400 output tokens. At Luna's output price that is about $0.0022. The same text from GPT-6 Sol would be about $0.044, from Claude Opus 5.5 about $0.088, and from GPT-6 Astra about $0.22.
Two caveats keep this honest. Reasoning tokens are billed as output and are not included, so real costs are higher. And in the ChatGPT app you pay with plan limits, not per token.
The real cost is the pictures
Text is cheap enough to ignore. Images are not. Turning even ten of those prompts into pictures in Ciyo at GPT Image 2.5 Flare, 4:5 and 1K is 20 credits, and a hundred would be 200 — for a hundred pictures that mostly start on a wooden counter.
So spend the prompt budget on variety, not volume. Ten deliberately different prompts will teach you more than a hundred near-copies.
Ciyo does not run GPT-6 Luna itself. Write the prompts in Luna, then paste the ones worth making into Ciyo's Image Generator.
A better request
Ask for fewer prompts with the variety written into the request: a list of settings to use once each, a ban on repeated openings, and a line that names the props you do not want.
Write 10 image prompts for one 4:5 photo of [product]. Use each of these settings exactly once: bakery counter, kitchen table, window ledge, picnic blanket, market stall, cooling rack, a customer's hands, a paper bag, a plate with one slice, a shelf at opening time. Do not start two prompts with the same word. Avoid wooden boards. Each prompt must say the image contains no words, letters or logos.One developer's view of Luna
Developers who build with the small models noticed the price cut first.
Posted 2026-09-22 by Simon Willison, an independent developer. He notes that GPT-6 Luna is half the price of GPT-5.6 Luna and calls Luna his favourite model for building product features. He is writing about software features, not image prompts.
GPT-6 Luna pricing
How much does GPT-6 Luna cost?
$0.10 per million input tokens and $0.50 per million output tokens in the API, as listed on September 22, 2026.
How much did 100 prompts cost?
About $0.0022 in output tokens by our estimate, before reasoning tokens. In the ChatGPT app it used plan limits instead.
Were the prompts any good?
All followed the rules, but they were repetitive: four openings covered all 100, and wooden surfaces dominated.
Can Ciyo run GPT-6 Luna?
No. Paste Luna's prompts into Ciyo's Image Generator; each 1K image with GPT Image 2.5 Flare cost 2 credits in our test.
Spend your budget on pictures that differ
Take ten varied prompts into Ciyo and compare the results on one board.