On top of that, NextJS doesn't even support the <Image/> component (and thus all their image optimization) when exporting to static output (unlike Gatsby). So you will be pushed even closer to pay them.
Also, the image optimization on the default Next.js server, for example deployed to a $4 VPS, also works without Vercel.
[0]: https://www.npmjs.com/package/next-image-export-optimizer