I'm assuming it cost somewhere between no review and a thorough one. Past that, I assume nothing. Pay reviewers per review or per hour like other consultants. Groups like Arxiv would, for a smaller fee, verify the reviewer's credentials and that the review happened.

That's if anyone wants the publishing to be closer to thr scientific method. Arxiv themselves might not attempt all of that. We can still hope for volunteers to review papers in a field with little, peer review. I just don't think we can call most of that science anymore.