Cost per approved banner: 0.5K drafts, 2K final, break-even at 1 in 2

Drafting at 0.5K ($0.075) and rendering only the approved banner at 2K ($0.15) beats rendering every candidate at 2K as soon as you approve fewer than one in two drafts. At exactly one in two the cost is $0.30 either way; at one in four it is $0.45 against $0.60.

The formula

With an approval rate of 1 in k: draft route = k x $0.075 + $0.15; direct 2K route = k x $0.15. Both are per approved banner, assuming the final is a fresh 2K generation of an approved draft, and that the final matches the draft well enough to be accepted.

The one number to measure

Your approval rate is the input. Count it across a real batch of 20 drafts before you pick the route: if you keep 12 of 20, render at 2K directly; if you keep 3 of 20, draft first.

def per_approved(k, draft=0.075, final=0.15):
    return k * draft + final

for k in (1, 2, 3, 4, 6, 10):
    print(k, round(per_approved(k), 3), round(k * 0.15, 3))

Gotchas

• A draft that doesn't look like its final is not a draft. Same prompt, same ratio, new generation: the 2K image can differ.
• Failed generations are not billed, so retries don't add cost.
• Use the same aspect_ratio for draft and final; 0.5K to 4K are the tiers Nano Banana 2.1 lists.