What n: 4 costs on each Sume image model: cost x n, billed table

On Sume's Images API you pay the per-image price times n. The docs say the endpoint pricing lines already include the margin, so you pay cost_usd x n. Four images cost four times one: $0.10 on Qwen Image, $0.40 on Nano Banana 2.1 at 1K, $0.75 on Nano Banana Pro at 1K.

Billed cost of n: 4

Caps that change the picture

The request cap for n is 10, but each model has its own ceiling, which the catalog publishes as the n range descriptor. Most rows list a maximum of 4. Grok Imagine lists 1, so four stills mean four calls at $0.025 each. Higgsfield Soul takes 1 or 4. Asking for more than a model allows is rejected, not trimmed, so read the descriptor first.

Failures and refunds

Billing is all or nothing. A completed generation is charged in full, a failed or cancelled one is not charged, and a client that disconnects early is treated as a failed generation. If you ask for four and the call fails, you pay nothing for it.

How to budget a batch

• Multiply the billed per-image price by images wanted, not by calls.
• Use n to reduce round trips; the price does not change versus single calls.
• Estimate from GET /v1/images/models/{id}/endpoints, then confirm with usage.cost on the first response.
• For ChatGPT Image 2.5, price depends on quality and size, so use the estimator values instead of the catalog display rate.

Per-call overhead and speed

Because the price is linear in n, the decision to batch is about time and complexity, not money. One call with n: 4 is one request to track and one job to poll; four calls are four jobs, four idempotency keys and four chances for a transient error. On the other hand, a four-image call takes longer to finish, so it is likelier to pass the 30-second sync wait and return 202.

A good default for a UI is n: 1 in sync mode for instant feedback and n: 4 in async mode when the user asks for options. For automation, prefer n: 4 with mode: "async" and one idempotency key per call. The total stays the same and your code has fewer jobs to follow.

Example

A set of 12 thumbnail options as three calls with n: 4 on Nano Banana 2.1 costs 3 x $0.40 = $1.20, the same as twelve single calls. Slow settings (4K, high quality, large n) may return 202, so branch on the status code, as the Image API page explains.