I just sold 16 pages worth of exploration data. I only got merits for pages that had at least one system worth 1 million credits (give or take). So while that may be the number of merits per credit you get, there appears to be some kind of threshold for which systems give you any merits at all (as has been mentioned in this thread).
Or is the requirement that the system must have a terraformable, water world or earth-like? (Indeed, all those 1+ million credit systems had at least one of those.)
It would be very Frontier areas to have some arcane hidden requirement they forgot to tell us; or someone made a change during testing and never reverted it.