The reason I wanted the Revivalist Church to cost gold (or maintenance) is that I think it needs some kind of cost beyond the opportunity cost of spending hammers to build it. Not because of the building itself, but because of the civic.
I don't like industrial or modern civics "obsoleting" ones from the previous era, because I don't feel they have in reality. Certainly when it comes to religion. However, from a gameplay perspective, newer civics should give you a reason to consider switching. I feel like a new civic should, if it's in keeping with my strategy for a game, make me seriously consider taking an anarchy hit to adopt it. It takes anarchy (or planning a Golden Age) to change to Intolerant, Secularized or Atheist, which is much more of an opportunity cost... and they seem kind of lame in comparison to State Church and Free Church.
Intolerant is more of a situational civic to run if you can afford anarchy before ramping up for war or in response to a wardec, so it has a role, if not one I've ever found necessary. Atheist gives a lot of beakers, although again it doesn't seem worth it compared to state religion benefits. (Why does it give all those beakers, anyway? The Soviet Union is the only enforced atheist society that made major scientific strides, and it still fell behind.)
Secularized... what is it for? Going back to AND and continuing into C2C, I never see AI civs adopt this, with good reason. +Culture and Happiness are its big selling points, and neither of those are hard to get without it.
I also don't care for the Secularized name, considering when it becomes available. Does it represent separation of church and state and freedom of religion in the 18th century, or secularization in the 20th century (mostly the latter half, at that)? As the techs are currently constituted, it seems like this should be Pluralism, and what's currently called Atheism changed to Secularized.
Re: Slavery, Patrician and gold. Tried going without Slavery in my last game, but I did adopt Patrician. I was still able to stay at 100% beakers almost all the way through to medieval and had 1000s of gold surplus. Monarch difficulty, C2C Continents map, but I had early lucky breaks with really nice goodie huts. Where maintenance costs are concerned, it seems like the big issue is how many cities you found or conquer.
In this case, I jumped out to three early because I got a Tribe from a hut and it taxed my economy, I stabilized, jumped out to six to secure my desired borders and it taxed my economy again. Then I sat on 6 as long as I could before having to worry about someone expanding by ship to the peninsula I'd blocked off. Moved up to 9, by then I had enough +gold buildings to soak the cost of expansion - it did push my gold into the negative until I could get +gold buildings off the ground in the new cities, but I had a large enough treasury to run at a deficit for a while.
If I'd gone to nine cities and filled up the available space as soon as possible, or attacked, I would definitely have broken the bank.
EDIT: My biggest complaint about Slavery is that for the longest time it's the only way to get use out of cottages! :U It's effectively +C on cottages in addition to its listed effects, because it's either Slavery or Barter until you get Coinage, which often comes much later. If Barter didn't have -C on cottages, Slavery wouldn't be the strictly better choice every game and it would depend on how you wanted to structure your economy.
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) give massive amounts of gold.
