GPT-6 Luna vs Gemini 3.8 Flash: API price comparison
GPT-6 Luna costs $0.10 per million input tokens and $0.50 per million output tokens. Gemini 3.8 Flash costs $0.75 and $3.75. For a typical support chat reply (1,500 tokens in, 300 out), GPT-6 Luna costs 87% less than Gemini 3.8 Flash.
| GPT-6 Luna | Gemini 3.8 Flash | |
|---|---|---|
| Provider | OpenAI | |
| Input, per 1M tokens | $0.10 | $0.75 |
| Output, per 1M tokens | $0.50 | $3.75 |
| Cached input, per 1M | $0.01 | $0.075 |
| Long prompts | $0.20 in, $0.75 out above the provider's threshold | Same price at any length |
| Context window | 1.05M tokens | 1.05M tokens |
| Reads | files, images, text | text, images, video, files, audio |
| A chat reply | $0.00030 | $0.0022 |
Gemini 3.8 Flash. Input: $0.75 per 1M tokens through December 31, 2026, then $1.50 from January 1, 2027.
Gemini 3.8 Flash. Output: $3.75 per 1M tokens through December 31, 2026, then $7.50 from January 1, 2027.
The same requests on both
What 10,000 of each request cost, at each provider's list price.
| Request | Tokens in / out | GPT-6 Luna | Gemini 3.8 Flash | Difference |
|---|---|---|---|---|
| A support chat reply | 1,500 / 300 | $3.00 per 10K | $22.50 per 10K | Gemini 3.8 Flash: 7.5x the price |
| Summarizing a 10-page document | 6,000 / 500 | $8.50 per 10K | $63.75 per 10K | Gemini 3.8 Flash: 7.5x the price |
| Sorting a support ticket into a category | 400 / 5 | $0.425 per 10K | $3.19 per 10K | Gemini 3.8 Flash: 7.5x the price |
| A question about a 120-page document | 60,000 / 800 | $64.00 per 10K | $480 per 10K | Gemini 3.8 Flash: 7.5x the price |
Which one should you use?
Price is one side of it. The other is whether the cheaper model answers your requests as well, and that depends on the request: a model that is good enough for sorting tickets or short summaries can fall short on long reasoning or exact formats.
The reliable way to know is to test both on your own requests, not on a benchmark. PromptUnit does that on a sample of your real traffic, and a step of your app moves to the cheaper model only once it keeps up (how it works).
More comparisons
Paying flagship prices for every request?
Most apps send simple requests to the same strong model as the hard ones. PromptUnit checks each request your app sends and uses a cheaper model only when it is just as good for that request. You pay 20% of what it saves, and nothing if it saves nothing.
Checked October 14, 2026. Prices are read every day from OpenAI's pricing page and Google's pricing page. Context window, inputs and release date come from OpenRouter's public model list.
These are the standard list prices on each provider's own API. Batch, flex and priority tiers, cloud platforms such as Amazon Bedrock or Google Vertex AI, and regional endpoints are priced differently. Request costs above count input and output tokens only, not cache, search or image fees.