One time after I successfully invaded a planet I noticed my funds dropped from +600 to -400 (I should mention that this invasion took place right after the colonization phase, so it was just as I was building my economy, which is why I only had 600 in my treasury). I thought this was strange since it was in the middle of a turn and I hadn't purchased anything so my money shouldn't be dropping. So I invaded another planet and my funds dropped 600 more. After I got out of debt I built some more transports and tried to invade some more planets. Again, every time I successfully invaded my treasury dropped by between 500 and 1000 each time.

In all games I've played since the same thing has happened. I just finished one in 1.2 and yet again, funds dropped after conquering.

I was just wondering if this is a bug or if successful invasions cost money. I've checked the literature and didn't see anything about it.