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If it is for the 100 richest people in the world, you'd probably just want to offer them a facilicy fee of USD25,000 a year. If the content is worth it, they'll get their people to pay your people.
If it is for subsistence farmers in Central Africa, then you need to check that Google has enough advertisers that address that market.
And so on.
if the forum covers a wider subject area then adsense may do better. you'll need to experiment with ad positions to get the best return.
if the content is good enough, then maybe some subscription forums could be introduced. bear in mind though that people need to know what the content is going to be like before they subscribe, so you'll need some sort of teaser content for it.