Nano Banana 2.1 vs GPT Image 2.5: 4.04x at 1K, 0.9x at 4K

On Sume, Nano Banana 2.1 is $0.10 per 1K image and GPT Image 2.5 on low quality is $0.02475, so Nano Banana costs 4.04 times as much at the bottom. At 4K the order reverses: Nano Banana 2.1 is $0.20 and GPT Image 2.5 on high is $0.2225, so GPT Image 2.5 costs 1.11 times as much. The two Sume ids are nano-banana-2.1 and gpt-image-2.5 on the Image Router.

The three matched points

GPT Image 2.5 is tiered by quality and size rather than by one resolution field, so the matched points below follow the Sume catalog rows: low at 1K, medium at 2K and high at 4K. Nano Banana 2.1 is priced per tier.

What the ratio means for a batch

For 200 images, the 1K gap is $20.00 against $4.95. At 2K it is $30.00 against $11.125. At 4K, Nano Banana 2.1 is $40.00 and GPT Image 2.5 on high is $44.50. GPT Image 2.5 wins on price at 1K and 2K, and Nano Banana 2.1 wins at 4K.

The quality tiers are not interchangeable. A low-quality GPT Image 2.5 draft and a high-quality final are different outputs, and this post makes no claim about which model looks better. Dense text and packaging are the cases where Sume's own docs suggest using a higher quality value.

Calling either one

Both ids go through the Images API with the same job envelope. Nano Banana 2.1 takes a resolution tier, and GPT Image 2.5 takes a quality value. The docs also note that the retired nano-banana-2 id still works and runs as 2.1.

curl -X POST https://api.sume.com/v1/images \
  -H "Authorization: Bearer $SUME_API_KEY" \
  -H "Content-Type: application/json" \
  -H "Idempotency-Key: nb21-4k-001" \
  -d '{
    "model": "google/nano-banana-2.1",
    "prompt": "Studio shot of a ceramic mug on a linen cloth",
    "resolution": "4K",
    "aspect_ratio": "4:5"
  }'

Rule of thumb

Draft on GPT Image 2.5 low at $0.02475, then generate finals at 4K on Nano Banana 2.1 at $0.20 if the 4K size is what you ship. Each of those two steps is a separate paid job.

Other sizes

Nano Banana 2.1 also has a 0.5K tier at $0.075. GPT Image 2.5 has no matching published row in the shared price list for 0.5K, so the grid starts at 1K. The catalog default estimate for GPT Image 2.5 is $0.2225, the high 4K figure, which is the number Sume reserves when you do not set a quality. If you leave quality at the Images API default for GPT Image 2.5, check the job estimate before a large batch.

Per-image prices are charged once per generated image, so asking for four images of one prompt multiplies every figure above by four. At 2K that is $0.60 for Nano Banana 2.1 and $0.2225 for GPT Image 2.5 medium.