I did the math on that 31 trillion market capitalization. Hehe, they'd need to sell about 12 billion of those 2500$ neural implants to get there.
Guess there's a monthly fee too!
Considering it was mentioned that Eternum was called "the drug of the rich," I highly doubt that Ulysses has managed to have sold that amount of neural implants.
I also don't think there's a monthly fee, otherwise Annie would've mentioned that to the MC at any point prior to him joining.
With that said, there's a lot of other possible ways one could do to monetize a game like Eternum:
-selling/renting out servers to people who want their own custom-built world (like how EA/Dice does it with Battlefield)
-selling worldbuilding tools to creators for a fee (or an annual license like how Microsoft does it)
-having an asset store where creators can buy and sell assets for a fee...hell, I wouldn't even be surprised if they have the latter as a sort of in-app store where they could sell hairstyles, costumes, weapon skins, etc. for those who want it (I mean, if I were the Founder I would add that...beats having a monthly fee to be honest).