You are right, must have misclicked somewhere for the lemmy.world stats. Here are the real ones: https://lemmy.fediverse.observer/lemmy.world
15472 monthly active users.
But your Reddit numbers are off by quite some margin. First, you are comparing Reddit's daily active users to lemmy.world's monthly active users. Reddit has 10^9 monthly active users (1.21 billion, to be exact).
Also, the ~โฌ2000 for lemmy.world are pure hosting costs (except of โฌ153 for donations), but for Reddit you included their whole revenue. That's not even their costs, that's their income.
I dug through their Earnings press release, and also there they don't specifically talk about their hosting costs. The closest I could find was "General and administrative" costs, which is what's left over of their total costs if you don't take "Cost of revenue", R&D and marketing into consideration, and that's $68.8 million per quarter (~$32 million per month), so 10^7, and that includes salaries and all sorts of other expenses down to the rent of the offices, the PCs their staff use and even the toilet paper. Hosting costs are at best a few percent of that figure, likely much less. So I'd knock that down to 10^6, likely even 10^5
That would give us $10^-4 to $10^-2 (if all administrative costs are purely hosting costs) per user. That's about the difference between paying โฌ2000 to host a Lemmy instance with 15k monthly active users and โฌ50 for hosting a phpBB instance to do the same.