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A runner-up witness (top 50) can produce between 20 and 45 blocks per day. At the current price of Steem, that is between $40 and $90/day, or between $1,200 and $2,700/month or between $12,000 and $32,400 per year (if I did the math correctly). As usual, witnesses / miners are betting on the price appreciation, so it's more of a medium / long term project.

Thanks for the clear explanation,... highly appreciated.

How does this cap work? There is at least one account in the top 19 which has less than 100,000 steem ( https://steemit.com/@klye/transfers ), and several in the top 50 that have less than 100,000. Are these account somehow grandfathered in, or how do they work at witnesses with less than the minimum?