The claim is that the service handles distribution across reliability, so I think the more interesting question is the odds of that _mechanism_ failing when your site would otherwise be up[1].

Similarly, 3-2-1 is a backup strategy and the pricing appears to already include multiple copies using the same mechanism so the correct calculation would be the cost of R2 plus whatever _different_ mechanism you choose for disaster recovery purposes such as on-premise storage or a completely different provider.

1. For example, if you use Cloudflare as your CDN / DNS provider and they have a catastrophic failure, the fact that your storage is inaccessible is just another facet of the same issue.