As usual for something so simple, Google's docs seem unclear: https://ai.google.dev/gemini-api/docs/pricing

For 3, pricing for image tokens was the same as text tokens. Since they don't indicate a difference on 3.7, I would assume the same holds. And as far as I know the number of image tokens is the same for both (depending on the detail level you pick, but it's generally around 1k per image).

So they're about the same, 3.7 is slightly more expensive. At least until the end of the year (when they raise 3.7's pricing).

Anyway, my point was that 3.5 tended to have worse performance and significantly higher costs. 3 and 3.7 are both better and cheaper than 3.5.