Hm.. the 'Do I get charged for creating and storing variants?' FAQ certainly is, but I was actually looking at the top one 'How much does Cloudflare Images cost?' which is a bit ambiguous:
> Cloudflare Images costs $5 per month per 100,000 stored images and $1 per 100,000 delivered images. Storage must be purchased in advance in blocks of 100,000 images.
> You only pay for original images; not variants. If you have one million original images and define five variants, you would only pay $50 per month for storing your one million original images.
On my first reading I took that to be general, but then the example is about storage... I think you're probably right, I was thinking each served image (or variant) would only be charged once per payment period (because they can cache aggressively) but that'd be really cheap, depending on usage.