Dropbox's fixed storage for a fixed price business model probably assumes a large majority of their customers wont use the full 1TB they pay for. Providing smaller amounts of storage for a lesser price would just reduce revenue without reducing storage costs, so it doesn't make sense for them.
If you want a pay per GB service, setup a S3 account (or similar) and get one of the many decent frontends to it. Its what I do.
Have you found decent frontends which let you sync files between Windows, Linux, Mac, Android, and iPhone, using S3 as the backend? If so, I'd love a few pointers, because I've been looking for something like that.
My personal use case involves files stored on a NAS, which is accessible via VPN and any client with a file browser (so, basically every platform). NAS is backed up via rclone to cloud storage. Its not sync, but I dont really want files synced to all my devices.
If you want a pay per GB service, setup a S3 account (or similar) and get one of the many decent frontends to it. Its what I do.