A user base as large as Reddit has an infra bill in the tens of millions. And that’s mature, with cost optimization at all levels to reduce compute, static content costs, more effective caching…etc
Lemmy instances are probably an order of magnitude more expensive to run on a per-user basis, at least.
This means the bill for the Lemmy fediverse if it had the active user base of reddit could be conceivably be near or over a collective $100mill/y with the majority of that just being a result of fragmented, high cost, infrastructure running a (at scale) low performance application.