I can't see where it can earn much revenue.
If it "does what it says on the tin" and is basically a list/database of forums, then can't see it being a very 'sticky' site. Once you've found a forum you like, you've no need to come back.
Only earning posibilities I can see are through adsence (or simular) or charging forum owners to list.
Therefore, I'd say £xx to reseller possibly low £xxx at the most to end user.