Lemmy is never going to handle an entire migration of reddit's whole user base. Most redditors use the official app, and the mainstream audience for the platform now represents the largest user group. They're not going to wholesale make the jump to Lemmy. That wouldn't even be possible without widespread coordination of resources. Each instance can only handle so many users, so new instances will have to continually be created to accommodate influx. Theres no profit incentive either, meaning whoever is running the instance server is purely doing it out of their passion for the platform. That doesn't scale linearly, there's only so many people out there with the resources to run a large instance.

Measuring the cost isn't possible. It depends on electricity and equipment costs which vary a lot. And the question doesn't make sense either.