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If they want to reduce the poll of mana, a possible way is a pay subscription which could be pay in mana (with a reduction if you pay in dollars compared to if you pay in mana, so paying directly is like a cost effective way to buy some mana).
It could also help to avoid too much people leaving together if the subscription is necessary to use the site.
Sorry, I misvoted by accident. In theory, since so far on average I make ~17,000 mana / 4 months = ~4250 mana/month = ~$42.50/month in potential charity donations, when factoring in funging effects of the Manifold Charity system, it might still be worth it at $4/month. In practice, I suspect spending aversion bias would make me not pay more than $1/month though.
@dionisos haha if it makes you feel any better it's only about 500 mana/month in market profits, the rest is daily/weekly bonuses, new trader bonuses, and bounties, which tbf might be about to suddenly all disappear so maybe my answer should be even lower lol
4-6 in theory, but in practice I wouldn't pay. There wouldn't be enough users and liquidity in most markets to make it fun.