Obviously there can be a cost involved with getting a city-state to ally with you, in the sense that you gift them gold to get reputation. But I noticed in my game the other night that allying with a city-state to get their resources appeared to cause my gold per turn to fall precipitously. Am I now paying maintenance on the roads that are in their territory? Are there other possible explanations?
Simple answer: No, not directly.
I played around with my save game, and the difference appears to be that as a result of going into "We love the king" mode, my gold from cities went down dramatically. I would guess that this is because the civilians were assigned to food instead of money to take advantage of the growth benefit. I guess this means you have to keep a close eye on how your civilians are assigned, especially later in the game when maintenance is high and money is a lot more important than a little population growth.