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GPT-6 Luna vs Gemini 3.8 Flash
A straight cost comparison of GPT-6 Luna (OpenAI) and Gemini 3.8 Flash (Google Gemini), per 1,000,000 tokens, verified October 2026.
Rate-by-rate comparison
| Metric | GPT-6 Luna | Gemini 3.8 Flash | Winner |
|---|---|---|---|
| Input / 1M | $0.10 | $0.75 | GPT-6 Luna |
| Cached input / 1M | $0.01 | $0.075 | GPT-6 Luna |
| Cache write / 1M | $0.125 | — | Gemini 3.8 Flash |
| Output / 1M | $0.50 | $3.75 | GPT-6 Luna |
| Context window | 1,050,000 | 1,000,000 | GPT-6 Luna |
| Batch API | Yes | Yes | Tie |
| Long-context tier | > 272,000 | > 200,000 | — |
For per-token metrics, the lower number wins. For context window and batch support, the higher or broader option wins. A dash means the provider does not publish that rate: a missing cache-write fee means writes are not billed per token, while a missing cached-input rate means cache hits bill at the full input rate.
Blended cost: 1M input + 250K output
| Model | Blended cost | Difference |
|---|---|---|
| GPT-6 Luna | $0.225 | Cheaper |
| Gemini 3.8 Flash | $1.69 | +$1.46 |
Ten workloads, two bills
The same two models, priced across ten workloads that differ only in their input:output ratio. Nothing else changes — no retries, no system prompt, no middleware.
| Workload | GPT-6 Luna | Gemini 3.8 Flash | Winner |
|---|---|---|---|
| Classification 100K reqs · 500 in / 10 out | $5.50 | $41.25 | GPT-6 Luna |
| Data extraction 10K reqs · 2K in / 200 out | $3 | $22.50 | GPT-6 Luna |
| Support chat 1K reqs · 4K in / 600 out | $0.70 | $5.25 | GPT-6 Luna |
| Code review 2K reqs · 8K in / 1.5K out | $3.10 | $23.25 | GPT-6 Luna |
| Summarisation 1K reqs · 20K in / 1K out | $2.50 | $18.75 | GPT-6 Luna |
| Agent loop 5K reqs · 12K in / 2K out | $11 | $82.50 | GPT-6 Luna |
| Translation 500 reqs · 15K in / 15K out | $4.50 | $33.75 | GPT-6 Luna |
| Long-document QA 200 reqs · 120K in / 800 out | $2.48 | $18.60 | GPT-6 Luna |
| Cached RAG 1K reqs · 50K in (45K cached) / 1K out | $1.46 | $10.91 | GPT-6 Luna |
| Nightly batch 10K reqs · 10K in / 2K out | $10 | $75 | GPT-6 Luna |
GPT-6 Luna wins 10 of 10, Gemini 3.8 Flash wins 0 of 10. The winner is set by the token mix, not by the headline rate.
Where the two bills cross over
GPT-6 Luna is cheaper on both sides of the mix: $0.10 vs $0.75 per million input tokens, and $0.50 vs $3.75 per million output. No input:output ratio reverses that, so on price alone the other model never becomes the better choice — it has to win on context, latency or output quality.
Cost at four scales
The same support-chat workload (4K input, 600 output per request) at four monthly volumes.
| Requests / month | GPT-6 Luna | Gemini 3.8 Flash | Difference |
|---|---|---|---|
| 100 | $0.07 | $0.525 | GPT-6 Luna cheaper by $0.455 |
| 1,000 | $0.70 | $5.25 | GPT-6 Luna cheaper by $4.55 |
| 10,000 | $7 | $52.50 | GPT-6 Luna cheaper by $45.50 |
| 100,000 | $70 | $525 | GPT-6 Luna cheaper by $455 |
What caching does to each bill
Caching only helps the part of the prompt that repeats. This is the support-chat workload (1,000 requests, 4,000 input tokens each) with an increasing share of that prompt served from a warm cache, written once on the first request.
| Cached share of the prompt | GPT-6 Luna | Gemini 3.8 Flash | Winner |
|---|---|---|---|
| 0% | $0.70 | $5.25 | GPT-6 Luna |
| 50% | $0.52 | $3.90 | GPT-6 Luna |
| 80% | $0.412 | $3.09 | GPT-6 Luna |
| 95% | $0.358 | $2.69 | GPT-6 Luna |
Both models publish a cached-read rate, so the discount is comparable: GPT-6 Luna reads cache at $0.01 per million, Gemini 3.8 Flash at $0.075.
Which one should you pick?
GPT-6 Luna wins on classification, data extraction, support chat, code review, summarisation, agent loop, translation, long-document qa, cached rag, nightly batch, while Gemini 3.8 Flash wins on none of the ten workloads above.
Beyond price, the two differ on input rate ($0.10 vs $0.75); output rate ($0.50 vs $3.75); cached-input rate ($0.01 vs $0.075); cache-write rate ($0.125 vs —); context window (1,050,000 vs 1,000,000).
Batch mode is available on GPT-6 Luna and Gemini 3.8 Flash, which halves the nightly job. Both publish cached-input discounts, so a stable system prompt is worth caching on either.
FAQ
Is GPT-6 Luna cheaper than Gemini 3.8 Flash?
On a blended 1M-input-plus-250K-output workload, GPT-6 Luna is cheaper by 87% ($0.225 vs $1.69). The gap changes for cache-heavy prompts and for batch jobs, as the workload table shows.
Which handles longer prompts, GPT-6 Luna or Gemini 3.8 Flash?
GPT-6 Luna has the larger context window (1,050,000 vs 1,000,000 tokens). Watch the long-context threshold on GPT-6 Luna and Gemini 3.8 Flash: rates rise once input passes it.
Do both models support prompt caching?
GPT-6 Luna bills cached reads at $0.01 per 1M; Gemini 3.8 Flash at $0.075.
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