Apologies for the dumb title, I just couldn't resist it.
I inadvertently stumbled upon a "hack" of sorts to take a planet for 0 AIP. To do this, you need to have hostile-to-all Marauders enabled, prefereably at a fairly (but not ridiculously) high intensity. It may take some time, but saving 20 AIP is nothing to scoff at.
1. Assemble a fleet and attack the planet you want to have. Clear out all the mobile units, guard posts etc. but do not touch the Command Station, Warp Gate or other AIP-incrementers.
2. Make sure you leave a scout or some form of monitoring. Check on the planet periodically and clear it out again if it rebuilds.
3. When you see Marauders attack, get a fleet ready. The moment the Command Station goes down, charge in and wipe out the Marauders before they manage to regroup. Take the planet.
And there, you got yourself a planet, and left the AIP bill to the Marauders.
Doing this several times can really screw over the Marauders faction, since they will face increased AI forces but won't be significantly more powerful themselves.
But it can end catastrophically for you if, for example, you can't wipe out the Marauders after they take the planet. And it costs more in the way of fleet forces, since you effectively have to conquer the planet twice.
This can, in theory, also be done with the Nanocaust, but for obvious reasons it is much riskier.
This is NOT a request to patch this; it's a neat little trick if pulled of correctly, but I find there are enough things that can go wrong that it won't unbalance the game too much.
What do you guys think?