The population could make it costly by exploiting the weakness of state-of-the art armies of the time, hence partisans. Today the population can make it costly by exploiting today's army's weaknesses, hence IEDs. Future populations can make it costly by exploiting future army's weaknesses, which will probably involve a lot of hiding and striking from where robots can't go, and possibly cracking systems.

Speaking of which, has anyone here played Deus Ex? Rewiring big drones surrounded by large amounts of troops to reverse their friend/enemy detection was the most fun I had in months.

Not that this is funny in real life, but the game is great at exploring how fighting that includes supersuits and drones could work.