Which AI API key is cheapest?

By the Zylo team · Updated June 8, 2026 · 4 min read

“Which AI API key is cheapest?” contains a hidden assumption worth correcting before anything else: the key itself is free. You are not charged for issuing or holding an API key on any reputable platform — a key is just a credential that identifies your requests. What costs money is the usage that flows through it: the tokens you send and receive when you call a model. So “cheapest API key” really means “lowest effective cost to run my workload through a key,” and that depends on two things you can actually control: the per-token rates you are billed and whether the platform adds a markup on top of them. Once you reframe the question this way, the answer stops being about hunting for a magically cheap credential and becomes about choosing cheap models and an honest billing model. The credential is never the expense; the inference is.

The key is free; usage is what you pay

Treat the key and the bill as two separate things. Getting a key should cost nothing and require no commitment — on Zylo you create an account with no credit card and generate a key for free, then keep it as long as you like at no charge. The meter only starts when you make calls, and it measures tokens: roughly the words and word-pieces in your prompts and the model’s replies. Input tokens and output tokens are usually priced differently, with output typically costing several times more. This is why two developers holding the exact same free key can have wildly different bills — one runs a few thousand short classification calls, another streams millions of long completions. The credential is identical; the usage is not. If you want the full breakdown of what actually shows up on an AI bill, how much AI API keys cost lays out every layer, and getting a free AI API key covers claiming one.

Where real cost comes from

Three layers determine your effective cost, and only the last two are within reach of a “cheap key” framing. First is the per-token rate of the model you call — this is the dominant factor, because the gap between a lightweight model and a flagship can be fifty times or more. Second is any markup the platform adds on every token, which scales directly with your usage and bites hardest exactly when your product grows. Third is any fee on loading credits or a monthly subscription, which behaves more like a fixed cost than a variable one. To compare two services honestly, add every layer that applies for a representative month and divide by the tokens you spent to get a true effective rate. Done this way, a tempting headline rate with a per-token markup often turns out more expensive at scale than a clear base rate with a one-time top-up fee. The deeper mechanics are in what is the cheapest AI API.

Cheap models do most of the work

The single biggest lever on cost is which model you call, not which provider you use. A wave of efficient models now delivers strong results for routine tasks — classification, extraction, routing, first-draft generation — at a tiny fraction of frontier prices. As of June 2026, low-cost models on Zylo include GPT-OSS 120B at about $0.039 input and $0.18 output per million tokens, DeepSeek V4 Flash at roughly $0.10 / $0.20, and Gemini 2.5 Flash Lite at $0.10 / $0.40 — point-in-time figures, so confirm against the live pricing page. Against a flagship at $5 to $30 per million output tokens, these can be fifty times cheaper, so a job that would cost hundreds on a flagship can cost single-digit dollars on the right small model. The practical habit is to default to a cheap model and reserve a premium one only for the reasoning-heavy steps that genuinely need it. Browse the model catalogue to compare rates per model.

Why one honest key wins

Putting it together, the cheapest setup is not a special discount key — it is one free key, billed at base rates with no markup, pointed at the cheapest model that does the job. Zylo is built for exactly that: the key costs nothing, every model is billed at its base per-token rate with no markup on usage, and the only platform fee is a flat 25 percent applied when you add credits, never on consumption — so your per-token cost never changes as volume grows. Because the same single key reaches both the cheapest open models and the frontier ones, you can route each request to the cheapest model that meets the bar without juggling multiple accounts or credentials. That makes the effective cost easy to predict in advance rather than something to reconcile after the fact. The honest answer to “which key is cheapest” is therefore: the free one, used wisely, at transparent rates.