You do have to pay to maintain CS. I wouldn't bother with patronage unless you plan to juggle a bunch of them and even then, I often end up with a large number of CS and end up paying full price, which isn't a problem usually. Kinda depends for me. If I'm warmongering or REXing and won't have a lot of SPs when patronage becomes availabe I usually skip it. For smaller empires who are going to have a tighter budget anyways, it can be a lot more handy. Going deep patronage and getting the flood of extra GP is nice too. One way to gain massive favor with a CS is to librate them. The AIs often go nuts conquering CS either for personal gain or to satisfy the "X wants Y eliminated" quests. As long as you don't mind a little war, liberating a CS puts them super high above ally status such that you won't have to pay them for a long, long time. There's also a SP in Order tree that causes your CS to decay faster. The AI can and will use this in the later game. There's a notification for it and it's very noticeable.