It would cost a tiny amount to state your opinion as an individual, but spamming an opinion would be more expensive. A problem is that deep pockets would allow you to fake wide support for a minority position, and that most of the money would just circle within an opinion group.

Ultimately I think we will come around to the idea of verified digital identities almost everywhere. You could still have an AI agent spam in your name (or pseudonym), but you could not pretend to be multiple people.