2K image price check: FLUX.3 $0.100, GPT 2.5 $0.0556, Banana $0.15

The short answer

At 2K, FLUX.3 Image costs $0.100 per image on Black Forest Labs' own API, GPT Image 2.5 at medium costs $0.055625 on Sume, and Nano Banana 2.1 costs $0.15 on Sume (and $0.0504 on Google's API). For 100 images that is $10.00, $5.56, $15.00 and $5.04. Sume does not list FLUX.3, so the FLUX.3 price is for BFL's service only.

The price table

Each figure is the vendor's or Sume's listed price for an image at about 2K. BFL defines its 2k class as about 4 megapixels, and Google defines Nano Banana 2.1's 2K as 2048 x 2048, which is 4.19 megapixels. Sume's GPT Image 2.5 point is the medium 2K figure from the shared list; GPT Image 2.5 prices vary by quality and size.

How to read it

Do not compare a vendor's own list with Sume's price as if they bought the same thing. Sume bills a per-image price to your wallet and returns hosted URLs, with one API across its image rows. The direct vendors give you their native features and rate limits. A fair comparison needs your own test of quality at the size you will use, since a cheaper model that needs two tries costs more than a dearer one that needs one.

Within Sume, GPT Image 2.5 at medium 2K is 2.7 times cheaper than Nano Banana 2.1 at 2K ($0.15 / $0.055625 = 2.7). It is not the same image, and GPT Image 2.5 has no 8:1 or 21:9 ratio, so the right pick depends on the shape you need.

Price per megapixel

A quick way to line up the sizes is price per megapixel. FLUX.3 at 2k is $0.100 for about 4 MP, or about $0.025 per MP. Nano Banana 2.1 on Sume at 2K is $0.15 for 4.19 MP (2048 x 2048), or $0.036 per MP. GPT Image 2.5 at medium 2K is $0.055625 for the same class of size, about $0.013 per MP. These are rough, since the size classes differ a little, but they show where the real gaps are.

Caveats

These three prices are not like for like. The sizes are classes: 2k on BFL is about 4 MP, and Sume's tiers are named sizes. The models differ in what they do well, so a price table cannot pick one for you.

Run your own prompt on all three where you can access them. FLUX.3 is only on BFL's side; the other two can be called through Sume.

Sume lists flux-2-pro at $0.0375 and flux-2-flex at $0.0625 per image at default options. They are FLUX.2, not FLUX.3, and they are priced at their default size, so quote the size you will use. Read https://api.sume.com/v1/catalog for the current list before you plan around FLUX.3.