Head to head · October 2026

GPT-5.6 Luna vs Gemini 3.5 Flash API pricing

GPT-5.6 Luna costs 7.0 times less per token at an 80/20 mix. Input costs 6.9x more on Gemini 3.5 Flash. Output costs 7.0x more on Gemini 3.5 Flash.

OpenAI

GPT-5.6 Luna

Chat products, summarisation, cheap tool calls.

Vendor: OpenAI
Type: Frontier
SayGM in / out: $0.15 / $0.89
OpenRouter in / out: $0.21 / $1.27
Blended, 80/20: $0.298
Per 100M tokens: $30
Output share of bill: 60%
SayGM vs OpenRouter: 29% lower

Google

Gemini 3.5 Flash

Fast multimodal tasks, high-throughput apps.

Vendor: Google
Type: Frontier
SayGM in / out: $1.04 / $6.26
OpenRouter in / out: $1.58 / $9.50
Blended, 80/20: $2.084
Per 100M tokens: $208
Output share of bill: 60%
SayGM vs OpenRouter: 34% lower

§ 01

Monthly bill at three usage levels

Both models on SayGM. The bars show how the price ratio turns into dollars as volume grows.

Fig. 1Monthly bill at three usage levels

GPT-5.6 Luna on SayGM
Gemini 3.5 Flash on SayGM

Side project5M in · 1M out

$1.64

$11

Production app50M in · 15M out

$21

$146

High volume500M in · 100M out

$164

$1,146

Show data as a table

	GPT-5.6 Luna on SayGM	Gemini 3.5 Flash on SayGM
Side project (5M in · 1M out)	$1.64	$11
Production app (50M in · 15M out)	$21	$146
High volume (500M in · 100M out)	$164	$1,146

Usage is millions of input and output tokens per month. No caching discount applied. Source: published rate cards, October 2026 snapshot. Current prices on saygm.com.

§ 02

Verdict

A common pattern is to send routine requests to GPT-5.6 Luna and escalate to Gemini 3.5 Flash on failure or low confidence. On one key, that is a model name change per request rather than a second provider account.

§ 03

Per-token prices

OpenRouter figures include its 5.5% top-up fee.

USD per million tokens, input and output. OpenRouter includes its 5.5% top-up fee. Snapshot from October 2026.
Model	SayGMin / out	OpenRouterin / out, fee incl.	SayGM vs OpenRouter80/20 blend
GPT-5.6 LunaOpenAI	$0.15 / $0.89	$0.21 / $1.27	29% lower
Gemini 3.5 FlashGoogle	$1.04 / $6.26	$1.58 / $9.50	34% lower

Current SayGM rates

§ 04

GPT-5.6 Luna vs Gemini 3.5 Flash questions

Which is cheaper, GPT-5.6 Luna or Gemini 3.5 Flash?

GPT-5.6 Luna is about 7.0 times cheaper than Gemini 3.5 Flash at an 80/20 input to output mix on SayGM, as of October 2026.

How much do GPT-5.6 Luna and Gemini 3.5 Flash cost on OpenRouter?

Including the 5.5 percent top-up fee, GPT-5.6 Luna costs $0.21 input and $1.27 output per million tokens, and Gemini 3.5 Flash costs $1.58 and $9.50.

Can I use GPT-5.6 Luna and Gemini 3.5 Flash with one API key?

Yes. SayGM serves both through their native APIs on a single key, so simple tasks can go to GPT-5.6 Luna and hard ones to Gemini 3.5 Flash, or both can run in a cascade model.

§ 05