LegacyFlo is not the service you will be using every month, or the same amount of data will be not migrating every month. It is a service which you will use some times or not for the rest of the time. That's why it is available only on the Pay Per Use agreement.
LegacyFlo service requirement depends on the activities you schedule at your end.
For example, at the time of deployment, when you create a new domain and need to migrate mailboxes from another mailing system, let's say from GSuite to SkyConnect. At that time, you will need LegacyFlo a lot.
Maybe at some point in time, you want to reduce your storage on SkyConnect, and you want to back up all that data on to S3 again, you will need the LegacyFlo service.