I'm pretty sure if I captured another city the cost wouldn't go up again. However, here I had no plans to capture any further cities so that wouldn't help. I'll try playing on a few more turns and report back, but it may take me a while before I can do that. No, I hadn't lost any cities before, although if I had (so that my current city count was less than my maximum city count) I believe the cost wouldn't have increased. I believe the costs are based on your highest number of cities ever held, but I didn't expect a city I immediately liberate to count against me.