oagersnap said:
I have an idea for civ 4: When a unit popped a goody hut, the gold woudn't just be added to your money total, but instead, it woud be displayed in the right click drop down menu, and woud first be added to your total, when (or if) the unit woud reach one of it's own cities. Maybe it could even be made so, that gold you have in one of your cities couldn't be used in another of your cities, unless they were connected by road or port/airport. A technology in the modern ages called something like "electronic money" woud disable the "city transferring problem", as there woud exist no "real" money, but only "electronic" money.
Actually carrying gold back to town is a little annoying. Transfering gold to the nearest city or a unit's "home" is realistic enough.
However, I like the idea of per-city gold storage for its historical accuracy -- the great empires tended to keep their wealth spread out at the provinces. It wasn't as if the whole Roman treasury was kept at Rome. That isn't to say they didn't extract tribute, but only a portion of the income transferred to the imperial city. If a nation wants to transfer wealth to another city or the capital, it should lose some percentage to the cost of transporation, risk of banditry, etc.
If cities had their own stockpiles, it would also make sense that barbarians (not to mention gold-starved rival civs) might attack the wealthiest cities -- players would need to be able to see or estimate a city's treasury, though. Thus, wealthy cities (for example, your capital) would require additional defenses.
As for the tech requirement -- I think banking would be sufficient. Banking would allow a Central Bank small wonder, the creation of which (in addition to a commerce boost for its home city) would allow you to transfer funds (not the actual gold, note, just a numeric representation) at no penalty. Electronic banking is just an enhanced version of the same basic concept.
Realistically (that is to say -- it might not actually translate to good gameplay), it would also allow you to fool around with monetary policy, like interest rates, bank insurance, gold standard, inflation/deflation, etc. Since Civ isn't a macroeconomic sim, that's probably not desirable or feasible.
Random, derived idea: "Wealth". Functioning somewhat like "Culture", wealth represents all the proceeds of commerce not redirected to taxes, luxuries, research, culture, etc. It's sort of a generic form of economic infrastructure (as opposed to the specific kinds represented by city enhancements and wonders). Once it reaches certain plateaus, the city gets some bonus to production and happiness. Any combat that can destroy buildings also reduces a city's wealth, which can drop it back to low levels. A city's owner could also grab some of its wealth for instant cast, though there are diminishing returns and it would cause unrest.