Google's Gemini API charges $0.134 per 1K or 2K image for Nano Banana Pro and about $0.0336 and $0.0504 for Nano Banana 2.1, so Pro costs 3.99x more at 1K, 2.66x more at 2K and 2.12x more at 4K.
Pro pays for itself only if Nano Banana 2.1 needs more than 2.66 attempts per usable 2K image, because $0.134 divided by $0.0504 is 2.66.
Batch pricing halves every rate, and a draft-then-final workflow of five 2.1 drafts and one Pro final costs $0.302 against $0.804 for six Pro images.