28-panel storyboard: 0.5K drafts, 6 Pro 4K key frames, $4.35

The short answer

A 28-panel 16:9 storyboard drafted at 0.5K on Nano Banana 2.1 costs $2.10 (28 x $0.075). Redoing 6 key frames at Nano Banana Pro 4K adds $2.25 (6 x $0.375). The plan totals $4.35, against $10.50 to render all 28 at Pro 4K.

If the panels only need to communicate staging and are not shown to a client, a GPT Image 2.5 low 1K pass costs $0.69 (28 x $0.02475), the cheapest route in this post.

How the pass is built

Send aspect_ratio 16:9 and resolution 0.5K on every draft panel. The 0.5K row ($0.075) is the cheapest banana tier in the catalog, so the draft model is 2.1. Sume prices the banana models by tier, not by ratio.

For continuity, pass the previous approved panel in input_references when you edit the next one. Reference images do not add a line to the banana rows.

Where the plan leaks

A second round of drafts for the 10 panels the director rejects adds 10 x $0.075 = $0.75. The total moves to $5.10. That is still under half the all-Pro-4K bill.

The key frames are re-rendered, not enlarged, so they may not match the 0.5K draft in composition. Use the draft as a layout reference and send it with the key-frame prompt.

A rule of thumb

The plan beats all-Pro-4K as long as drafts plus key frames stay under $10.50. Each extra key frame adds $0.375, so the limit is ($10.50 - $2.10) / $0.375 = 22.4 key frames, which is nearly the whole set. With 6 of 28 you are far below it.

Read usage.cost on each response to confirm the billed price. Slow 4K calls can degrade to a 202 job; poll the result URL rather than resubmitting.

Keeping continuity between panels

A storyboard needs the same characters and set across panels. The cheapest way to hold continuity is to send the last approved panel as a reference with each new draft, and to name the character's clothing in every prompt. References do not add to the banana rows, so this costs nothing extra on 2.1 drafts.

If you send each draft as its own call, 28 calls make the pass. If you want fewer round trips, batch panels of the same scene with n=2 and pick the better one: 14 calls at n=2 cost the same $2.10, but you keep only half of what you paid for.

Finally, record which 6 panels carry the story beats. Choose the key frames before you spend the $2.25, not after.

Slow 4K calls can return a 202 job envelope instead of an image. Poll the status URL and fetch the result rather than resubmitting, because a second submit is a second charge once both complete.