I'd say around 500-800 depending on NPCs.
But that's irrelevant since undermining is supposed to be easier, otherwise no power would ever go in turmoil, ever. The week when Arissa was undermined for the first time is a testament to that. Not sure which federal groups decided to do it instead of backstabbing Sirius, but they were good at it. Then the Arissa supporters just threw credits from the huge piles sitting there from their insane bonuses and avoided turmoil.
Now imagine if fortification got a buff. Everyone would be Arissa. I imagine Hudson finds himself in similar position, with his supporters having some nice piles sitting, ready to hasten quotas. Although, admittedly, they are brought to using them more often, since the Empire is attacking them but they don't reply back properly.
Half the PP players are already Arissa because she gets bonuses on two of the most popular activities and best BB rewarded missions in the game, that is Combat and Smuggling when she has plentiful access to Slaves. That's the reason, not the merit reward balance.
Undermining and Expansion are well rewarded, fortification is not, is a chore, can involve players waiting for stocks to supply (usually avoided by doing a local mission) so why not let players get some extra merit candy while they wait, variety in options is always good.
Killing those in a Home system won't affect PP (it can't), only give players something to do while they wait for the next batch to drop because not everyone can afford to throw money at it. The only current alternative is to bb mission and if I trade more fish I will probably cry.
Any of these kills in a Control System however, should count towards PP by way of Fortification as that's pretty much what you're doing, taking out the trash and 5 merits is not an awful lot. It would be something you would do on your way in or out, just a nice bonus because as mentioned above, there's a clear disparity between what Fortification gains and Undermining and it's ridiculous currently.
The benefits, if tied to the BGS would mean less interdictions for those returning to pick up more. This would make the whole thing make more sense and players might actually feel like they're doing some good. It adds depth to one simple thing, protecting your systems which should be important.
When one thing is easier than another then people flock to the easiest way possible, that is time honoured in gaming and why everyone eventually hits turmoil every 3 or so weeks, maybe squeeze through to the 4th then turmoil again. They're chasing the merits for the rewards and not playing the game and that is bad.
The imbalance between Fortication and Undermining / Expansion are key factors in this.
If you want it balanced then Fortification needs to be as viable as Expansion and Undermining. It really is that simple.
I'm not opposed to Undermining being promoted as the best way to overthrow a power but the current gap between it and Fortification is far too much. Especially in how it rewards a player.
When all things are equal then players will start thinking about the game and not so much about where the best rewards are to be made.
Then you have player interaction on a level beyond solely merit farming.
Then FD can start thinking about levels of undermining on systems and where that goes.
They could also consider capping systems that are clearly being exploited by Fortification farmers to stop that.
Then players may actually start playing the game, rather than just exploiting it for the 50 million.
And that brings me to the piles of money thing you mention, the rewards need stepped back a bit, the jump from 5 to 50 million is way too much.
I can see this going down well but what do.
Also, Fortification should be easier, it should be harder to Undermine, and even if fortified, if there's a concentrated effort on a system then it should invoke some kinda "war / power struggle" when the threshold is met.
Just thoughts.