The biggest cost for us on Vercel (several hundred dollars a month) was Image Optimization, and that was because the app was being majorly inefficient with images, in part due to some default behavior in Next.js that we found unfriendly [0], and in part due to negligence. That being said, it wasn't "cheap" by any means outside of that, still hundreds a month for something that I would not consider a high traffic application (I wish I could remember more specific numbers).

Migrating to OpenNext using SST, I think we got the bills for compute and asset serving down to like $15/day or something (granted, we spent expensive engineer time on the migration).

[0] https://nextjs.org/docs/app/api-reference/components/image#s...