A direct cost comparison between two popular mid-tier LLM APIs in 2026. Headline rates, per-call math, and the trade-offs that matter beyond price.
On a standard 1,500-input / 500-output token workload, GPT-5.4 Mini is about 61% cheaper than Gemini 3.5 Flash — $0.0026 versus $0.0068 per call. At 10,000 calls per month that's roughly $26.25 vs $67.50; at a million calls a month, $2,625 vs $6,750.
Unlike many match-ups, this one doesn't flip with your input/output ratio: GPT-5.4 Mini is cheaper on both the input rate ($0.75 vs $1.50) and the output rate ($3.00 vs $9.00). The more output-heavy your workload, the wider the gap grows, because Gemini's $9 output rate is 3× higher.
| Metric | Gemini 3.5 Flash | GPT-5.4 Mini |
|---|---|---|
| Vendor | OpenAI | |
| Input price per M tokens | $1.50 | $0.75 |
| Output price per M tokens | $9.00 | $3.00 |
| Context window | 1M | 1M |
| Output/input ratio | 6.0× | 4.0× |
| Cost per typical call | $0.0068 | $0.0026 |
| Cost per 10,000 calls | $67.50 | $26.25 |
If price is the only axis, GPT-5.4 Mini wins outright. But the two models tokenize differently, and that changes the effective cost on non-English text. Google trained Gemini's tokenizer on a more multilingual corpus, so Russian, CJK and Arabic content is split into fewer tokens than on the GPT family. For a heavily multilingual product, that efficiency can erase — or even reverse — GPT-5.4 Mini's headline advantage. Price your real prompts in the language you actually serve before deciding.
As always, the strongest production pattern is two-tier routing: send the easy 70–80% of requests to a budget model like one of these, and escalate only the hard cases to a flagship. That beats single-model setups on both cost and quality.
Both Google and OpenAI support prompt caching (cached input bills at roughly 10% of the standard rate) and batch processing (about 50% off for 24-hour-tolerant jobs). If your app reuses a long static prefix, caching can shift the comparison — measure it on your own traffic rather than the sticker price.
For English-dominant workloads where price decides, GPT-5.4 Mini is the clear pick — cheaper on every rate. Reach for Gemini 3.5 Flash when its multilingual tokenizer or your own quality evals justify the premium. Run the numbers on your real prompts with the calculator.
Yes — cheaper on both rates ($0.75/$3.00 vs $1.50/$9.00 per million). A typical 1,500/500 call costs $0.0026 vs $0.0068 (~61% less), and it stays cheaper for any input/output mix.
When you serve non-English (Russian, CJK, Arabic) traffic — its tokenizer uses fewer tokens for those scripts, which can offset GPT-5.4 Mini's lower price.
On the standard 1,500/500 workload: about $26.25/month for GPT-5.4 Mini and $67.50/month for Gemini 3.5 Flash.