640 transparent icons: background field $15.84, plus RMBG $30.24

For 640 transparent icons, ask GPT Image 2.5 for background: "transparent" at low quality: 640 x $0.02475 = $15.84. Generating with any other model and then removing the background adds $0.0225 per icon, which makes the cheapest alternative, Grok Image at $0.025, cost 640 x $0.0475 = $30.40.

Which models have the field

The Image API says background: auto|transparent|opaque is supported on ChatGPT Image 2.5, so openai/gpt-image-2.5 and openai/gpt-image-2.5-sunburst. Other rows do not list it. A request that sends a parameter a model does not list gets 400 unsupported_parameter, so choose the model first.

For models without the field, remove-background is the route. It is $0.0225 per image and returns a PNG with alpha.

The trade-offs

The native field is the cheapest, but the table treats all outputs as equal. They are not. A model that draws on a transparent canvas may leave soft semi-transparent fringes or a faint halo on dark backdrops. A cutout works from a finished image and may cut tighter. Test 10 icons both ways on the background colors your app uses.

Icons have a second concern, which is consistency. If 640 icons come from one model at one quality with one style prompt, they look like a set. Mixing routes to save a few dollars can cost more in design review time than it saves.

Checking the alpha

Do not trust a file extension. Open each PNG in code and check that the alpha channel has values below 255 in the corners. A short Pillow check over the folder finds the icons that came back opaque so you re-run only those. The cost of 20 re-runs at low quality is 20 x $0.02475 = $0.495.

If an icon file already has transparency and you send it to RMBG, the gray-flatten note explains what Sume does with it. For the order of cutout and upscale on icons that need a larger size, see the order guide.

Batch plan

Send icons in groups of 64, ten groups in all, and review one icon from each group as it finishes. Ten groups at $1.584 each (64 x $0.02475) give you ten natural stop points where a prompt drift costs under two dollars.

Use the same style words in every prompt, such as flat, 2 px outline, single object, centered. Vary only the subject noun. That keeps a set of 640 reading as one family, whichever route makes the transparency.