Mana Confluence VS City of Brass
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, meaning you can essentially run 8 copies of essentially 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. Mana Confluence only pains you when for tapping mana. Basically, Mana Confluence is 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 just in case.
Mana Confluence in Modern and Other Eternal Formats
After enjoying quite a bit of play in Standard, Mana Confluence really only sees play in Dredge decks in Modern. It also sees play in dome Green/White Humans decks, and the occasional Allies deck. Interestingly enough, some Ad Nauseum combo lists play City of Brass over Mana Confluence, and it's not really clear as to why. Mana Confluence also sees play in Legacy Dredge, and the occasional Vintage deck in Magic Online.
Mana Confluence is definitely worthy of Commander play, and according to EDHERC, sees slightly more play than City of Brass. Unsurprisingly, both usually see play in the same decks. With 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.
Mana Confluence Price and Speculation
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 has fairly steadily crept up from $5 to $7 since it rotated from Standard in October of 2015. The funny thing is that it was reprinted as a Zendikar Expedition in Oath of the Gatewatch, just a few months after losing its Standard eligibility. The Expedition started out at $87.99 before falling to around $48. Both are solid pickups going forward, especially the Expedition version. 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, and will see play in Commander for many, many years to come.