I'd been asking myself a similar question lately. This is the best I could do. Market cap fro SME is currently approximately $400,000. There are 1044 users who have linked their Hive accounts and are participating in the rewards system out of 55k total users we're told SoMee has. That works out to $383 per user that is participating in rewards. If that same ratio held to 5M users, 90,000 users would be participating in rewards, with $383 of market cap per user, that would be an SME market cap of $34.4M for a token price of $.0172 per SME. You;d have to earn 145,000 ish SME tokens per month at that price to make $2500 of income. 69M rewards tokens are released each month. Divide that by your portion of the total circulating supply of staked SME to get your montly portion of that. Not perfect as the variables around curation strategy and post/comment rewards are unknown, but that is the best I could do.
RE: Let's assume we reach 5M users. How much SME do you need to have a stable income of $2,500