City of Brass VS Mana Confluence
With City of Brass, anytime it became tapped, you took one damage. However, with Mana Confluence, you only take damage when you actually tap it for mana. While this is a very minor difference, it does technically make it very slightly better. It also means that there are now functionally two Cities of Brass in existence. This means you can essentially run 8 copies of the same card in one Constructed deck.
While paying 1 life to color-fix can sound like a lot, keep in mind that City of Brass has been used for a very long time in multiple formats. It's still used in some fringe Modern decks. Also, anything that would tap down lands would force you to take 1 damage.
However, Mana Confluence only pains you when for tapping mana. Basically, it's a five-color pain land. However, it doesn't have the ability to tap for colorless mana without a life cost, such as the traditional "pain lands" from Ice Age/Apocalypse/Tenth Edition.
There's another case in which Mana Confluence is strictly better than City of Brass. Urborg, Tomb of Yawgmoth, which makes all lands in play Swamps in addition to their other types, was in Standard at the same time as Mana Confluence. This is great, because Urborg gives Mana Confluence the additional ability to tap for Black mana. Because it's an additional ability, you can tap it for Black mana without paying 1 life. While this is now a corner case ruling, it's important to remember.
Mana Confluence in Pioneer, Modern, and EDH
After enjoying quite a bit of play in Standard, Mana Confluence continued to see play in Legacy Dredge decks. It also saw play in some Modern Green/White Humans decks and the occasional Allies deck. Interestingly enough, some Ad Nauseum combo lists would play City of Brass over Mana Confluence, and it's not really clear as to why.
With the introduction of the Pioneer format in October 2019, Mana Confluence began to see some renewed play. Because there are no fetch-lands available in Pioneer (a Modern format that begins with Return to Ravnica and has a separate banlist including the Khans fetches), mana fixing is at a premium. However, Mana Confluence typically only sees play as a singleton in a few decks: Niv to Light, Gruul Aggro, and Golgari Stompy. Still, the renewed interest in the card is a good thing, especially in a format that lacks City of Brass.
Mana Confluence is definitely worthy of EDH / Commander play. According to EDHREC, Confleunce sees slightly more play than City of Brass, but by a miniscule margin as of April 2020. Unsurprisingly, both usually see play in the same decks.
With the release of Commander 2016's four-color commanders, Mana Confluence saw a slight rise in play. Atraxa, Praetor's Voice, Breya, Etherium Shaper, and Yidris, Maelstrom Wielder are the three Commanders who utilize Confluence the most. Even with four and five color decks becoming easier to play as time goes on, a five-color land like Mana Confluence will always find a home.
Mana Confluence Price History
As a solid mana-fixer and a functionally superior version of an existing land, it's not surprising that Mana Confluence is a card with a fair amount of demand. Most of that demand comes from Commander, which is also not surprising, as it can fit in pretty much any three-color, four-color, or five-color EDH deck.
The price of Mana Confluence steadily crept up from $5 to $8 after it rotated from Standard in October of 2015. But, in early 2019, original printings of Mana Confluence crept up to $9-10 before jumping to $15 in April. The card then doubled in price almost overnight with the late October 2019 announcement of the Pioneer format. In mid-2020, it was holding a $25 price point.
Mana Confluence was reprinted as a Zendikar Expedition in Oath of the Gatewatch, interestingly just a few months after losing its Standard eligibility. The Expedition started out at $87.99 before falling to around $48. But in early 2018, there was a buyout of the Expedition, which caused to price to shoot through the roof before it stabilized at $125. The price gradually settled back to around $100, shooting back up to $125 and steadily returning to $100.
Both versions of this land are solid pickups going forward, especially the Expedition version. It's true that Mana Confluence could be reprinted at some point. But, even if it is, there's plenty of demand for it, since it fits into such a wide variety of decks.
It's actually a bit surprising that we don't see this card more in Modern than we do. But, in any case, this land is superior to City of Brass. If nowhere else, Mana Confluence will see play in EDH for many, many years to come.