40 Imagen 4 Fast drafts, then a Nano Banana 2.1 edit: $1.10

A cheap two-stage flow on Sume is to draft 40 options on Imagen 4 Fast, pick one, and refine it with a single Nano Banana 2.1 edit. The cost is 40 x $0.025 = $1.00 for the drafts plus $0.10 for the edit at 1K, so $1.10 in total.

How the hand-off works

Imagen 4 Fast is text-to-image only on Sume, so it cannot refine its own output. Sume returns Sume-hosted URLs for results, and Nano Banana 2.1 accepts public HTTPS reference URLs, up to 10. Pass the chosen draft URL in input_references with a prompt that states the fix: correct the logo, change the wall color, remove the cup. Use the Sume media URL that the job returns.

Limits to plan for

Imagen 4 lists five aspect ratios: 1:1, 16:9, 9:16, 4:3 and 3:4. If the final needs 4:5, 4:1 or 8:1, draft in a close ratio and let the Nano Banana 2.1 pass set the final one with its own ratio list, which includes 4:5, 4:1 and 8:1. Note that Google's Imagen page says Imagen is shut down in the Gemini API; check that the Sume row still lists, or swap the draft stage to another cheap row.

When not to bother

If the final will be a photo edit anyway, skip the draft stage and send the real photo to Nano Banana 2.1. The two-stage flow is for ideas from a blank page.

Cost control

Imagen 4 Fast is the cheapest row in the set, so 40 drafts cost the same as 10 images on Nano Banana 2.1 at 1K ($1.00 either way). That is the argument for the flow: for a dollar you see 40 ideas rather than 10.

If you want twice the ideas, 80 drafts cost $2.00. The refine step stays at one call. The refine is the part that depends on the model: at 2K the edit costs $0.15 and at 4K it costs $0.20, so a 4K refine after 40 drafts totals $1.20.

Log the prompt of the winning draft. It is the best prompt you have and the refine prompt should reuse its subject words.

Quality of the hand-off

Check the picked draft at full size before you spend on the refine. If the composition is wrong, the edit will not fix it. If only details are wrong, the edit is the right tool: a wrong logo, an extra object or a color shift.

Also note the cost of rejected refines. If the first edit fails, a second one costs another $0.10. Even with three attempts the flow totals $1.30, the same as 13 Nano Banana 2.1 images at 1K.

For a blog or social team, the same flow scales to a weekly budget. Ten posts a week at $1.10 each is $11.00 a week, or about $47.63 over a 4.33-week month (11 x 4.33). Compare that with generating each post's image once at Nano Banana 2.1 1K for $0.10: $4.33 a month. The flow costs more because it buys choice, so use it where choice matters, such as ads, covers and hero art, and skip it for filler images.