Head-to-head showdown: Gemini 3.1 Pro ($2.00 in / $12.00 out per 1M) vs Gemini 3.5 Flash (OpenRouter) ($1.50 in / $9.00 out per 1M). Gemini 3.5 Flash (OpenRouter) is 1.3× cheaper across standard token mixes, with 2M vs 1M context windows.
gemini-3.1-pro · google
gemini-3.5-flash · google
Benchmark
| Workload Scenario | Gemini 3.1 Pro | Gemini 3.5 Flash (OpenRouter) | Price Delta |
|---|---|---|---|
| 1M input tokens (raw text) | $2.00 | $1.50 | +$0.50 |
| 1M output tokens (generation) | $12.00 | $9.00 | +$3.00 |
| 1M tokens · 70% input / 30% output mix | $5.00 | $3.75 | +$1.25 |
| Standard chat turn (4K in / 800 out, 50% cached) | $0.014 | $0.0105 | +$0.0035 |
| Monthly scale (10K requests / day) | $4,200.00 | $3,150.00 | +$1,050.00 |
Negative difference = Gemini 3.1 Pro is cheaper. Positive = Gemini 3.5 Flash (OpenRouter) is cheaper.
Scaling Curve
Total cost of a token volume at 70/30 input/output split (uncached). Log-log scale.
Verdict
Gemini 3.5 Flash (OpenRouter) is cheaper on both input and output rates, so it costs less at every input/output mix. Price alone still isn't the whole decision: capability, latency and context limits (Gemini 3.1 Pro: 2M, Gemini 3.5 Flash (OpenRouter): 1M) may justify the premium for your task.
FAQ
On input, Gemini 3.5 Flash (OpenRouter) is cheaper ($1.50/M vs $2.00/M). On output, Gemini 3.5 Flash (OpenRouter) is cheaper ($9.00/M vs $12.00/M). The same model is cheaper on both sides, so it wins at every mix.
A chat-style request (4,000 input + 800 output tokens, 50% cached) costs $0.014 on Gemini 3.1 Pro and $0.0105 on Gemini 3.5 Flash (OpenRouter) — Gemini 3.5 Flash (OpenRouter) is 1.3× more expensive for that workload.
Gemini 3.1 Pro supports 2,000,000 tokens (128K max output); Gemini 3.5 Flash (OpenRouter) supports 1,048,576 (65.5K max output). Gemini 3.1 Pro fits 1.9× more context, which matters for long documents and agents.