So, I've been looking at the early Economics Civics, I've put a fair amount of thought into it, and it seems to me that the first three; Barter, Slavery, and Coinage, are fairly underserved at the moment. The concepts for which these 3 Civics are the earliest representations of are fundamental paradigms of human exchange that are currently not really done justice. I'll try to express this in as organized a manner as I can.
Viability Issues
Coinage: I'm going to start with Coinage simply because it is, as far as I can tell, just so bad. This unless I'm missing something huge this civic is not only underpowered, its actively detrimental. The main reason for this is the fact that it consumes all Gold and Silver resource for a measly 1 and 0.5

per city respectively. Compare that to the bonus that the Jewelry Building gives for the same resources; 1

each. That 1

for the Gold resource is superior because it benefits from +% to

before receiving +% to

in contrast to the Coinage civic, and the 1

for the Silver resource is superior for the same reason, and because 1 > 0.5. This consumption effect also cripples the
National Mint that Coinage has access to later. To use the local lingo, the only real 'trick' the civic has left is +20% from domestic

, though this is offset by -20% from foreign

.
Barter: This civic is currently being treated as a stepping stone civic in the same way that most of the other starter civics are. The player and ai are, because of the massive penalties associated with Barter and because of the placement of Techs, funneled into slavery basically ASAP.
Because Barter and Coinage are so bad, the player and AI have really no viable options besides Slavery until Guilds, halfway into the Medieval Era. This doesnt really make game sense, or historical sense.
The awkward position of Slavery
Slavery as a civic is in an awkward position because slavery, serfdom, and otherwise denominated human servitude have been with us for so long, since some of our earliest records of law
[1] to the last vestiges of serfdom as late as 1923 in Afganistan, to the fact that forced labour still exists today. Yet many of the economies throughout history that have allowed slavery(most of them) did not mainly consist of slavery, and were not distinguished by their slavery. Slave labour may have been important, yet free labour, artisans, and other components were equally or considerably more important. This is why I would argue that a Civ with the Slavery Civic represents not an economy that has slavery, but an
economy largely based on slavery. This semantic distinction allows us to treat the Barter civic as a viable early alternative to slavery, rather than as an extremely penalizing stepping stone.
Thoughts and Suggestions for Barter/Slavery
Barter
A. My main suggestion for Barter is to buff it until its a somewhat viable alternative to slavery, and possibly to nerf slavery. I think the massive penalties to both

and

for barter should be at least reduced to perhaps -10% or -15%.
B. My second suggestion for barter is to give it small

bonuses for Buildings that require a resource in city vicinity
(Furrier), or in some cases resources themselves
(wheat and resources without city-specific buildings?) or early buildings
(perhaps carpenter). Firstly this would allow barter to scale a little further into the ancient era, as the player researches the early techs barter would gain a little bit of extra

to offset the penalty. Secondly this would fit Barter thematically, because even though barter based economies
did in fact trade frequently using standardized weights of precious metals, basically proto-coins it remains that diversity of available goods is especially helpful to a barter economy.
C. My third suggestion might be a little trickier, and it has to do with the differentiation of the Slavery Civic as a slave based economy, not just a society with slaves. I rather recommend that Barter, and perhaps Coinage, allow for each city to use slave specialists in a limited way. I would suggest either tying this to a building that is unlocked by researching the Slavery tech, or I would suggest allowing at least Barter to build unlimited Slave Specialists, but give their slaves -

or some other penalty so that they are usable, but not optimal.
D. I'm also testing making Barter into at least Low Upkeep rather than None, no upkeep can be too valuable later in the game, but low upkeep is 0 early on when Barter is intended to work so this helps incentivize leaving Barter later on without damaging it early.
Slavery
Some people have argued that Slavery is still too strong of a Civic, and in some very strategic ways it is, which is I believe why we currently have the Slave Market going obsolete in the modern era. I would suggest that we could adjust Slavery a bit, and perhaps nerf it, in a few subtle ways:
A. Rather than having the Slave Market go obsolete, have Slavery receive a significant penalty to Foreign

. This represents thematically the incompatibility between heavily slave based economies and non-slaving societies. Remember that although the vast majority of historical societies had some kind of institutionalized servitude, many of them did not support actual slave trade
(they may have had serfs but serfs had rights and could not be bought or sold etc., there are many examples). The other effect of this that I believe we desire for gameplay reasons is to give both the player and the AI reasons to switch out of Slavery as the game progresses, because Foreign

scales so hard in the late game.
B. My second thought about Slavery is that along with it's bonus to

it should perhaps receive a penalty to

. In the end this adjustment could be a nerf, or the math could be worked so that it is not a nerf, but thematically I think it could be argued that a Slave based economy would likely not have as much technological innovation as other economies.
C. Perhaps a penalty to relations with other Civs who are not using Slavery, this is another thematically supportable avenue to nerf Slavery in the later eras.
Coinage
The advent of specie currency in various places in the world is very interesting, and the introduction of coinage into the Mediterranean economy was absolutely fascinating
(Mediterranean societies continued to use barter and slavery alongside their coinage btw), but frankly, most of the really fun things (debasement, influx of resource, deflationary spirals) that could be done with Coinage in a game are I think not really supportable within the Civ4 framework, the AI could not handle it.
A. My suggestion then is to use the fact that the long-term trend of Coinage is that of Inflation and Debasement, which combine into the concept of Seigniorage[2]. In game terms, the theme of Coinage would look something like a +2x%
along with a -1x%
.
B. My second suggestion is that Coinage should be at least Medium Upkeep, minting coins is not free, and this would encourage larger civs to abandon the Civic later in the game.
C. My third and strongest suggestion is to remove the XML that causes Coinage to consume all Gold and Silver. If we could make it consume just 1 of each I think that would be OK, but having Coinage just remove all of those resources from your empire screws with too many other buildings, bonuses, and mechanics.
I think that combined, my suggestions would be at least a step toward much more interesting and balanced early game Economics Civics. I've tried to be general enough when it comes to balance concepts to leave room for improvement. I hope my thoughts have at least been helpful.
1. https://en.wikipedia.org/wiki/Code_of_Ur-Nammu
2. https://en.wikipedia.org/wiki/Seigniorage