I was OK at first with the whole decimal system of currency that they have in 3e, but the more we play, and the higher the characters get, the less I like it.
My biggest problem is the lack of complexity and general transparency across cultures/governments leaves the flavor pretty bland.
About 20 years ago, I felt much as you. The truth is though that adding culture in the form of diverse coinage doesn't add much value to the game compared to the trouble it creates in book keeping. RPing money changers, while very realistic, just isn't very interesting. Nor is keeping track of the dozens of sorts of coinage that a person might loot.
What was the relative rarity of the classic precious metals (copper, silver, gold, platinum), using medieval/fantasy mining methods in the Old World? What if the same culture/technology developed, but on the continent of North America? What early colonial exploration (Africa, India, Central America, China) generated booms (and presumably deflation of value) of what metals? (e.g. I know scads of gold came from looting the Aztecs).
If medieval society developed on the continent of North America, would the currency be based on silver?
Yes. There isn't enough gold in the real world to base common coinage off of gold. Historically, the ratio of silver to gold varied depending on the productivity of mining techniques and availability of ores, but Gygax's ratio of 20:1 silver to gold is in my opinion closer to the norm than 3e's simplified decimal ratios. Historically, platinum was unknown as an individual metal, but found alloyed with gold. Presumably platinum coinage is of dwarven origin, given their superior metalworking, it would make sense if they discovered and utilized it. Gygax's 5:1 ratio is less based on history, than the price of platinum relative to gold in the 1970's.
Copper is a 'base' metal rather than a precious metal, and copper coinage has very little intrinsic value. Generally speaking, the metal in a copper coin is worth less than its denominated value which means no one prefers to take payment in copper if they have choice in the matter.
I generally simplify the game by having coins have a standard weight used by all cultures, and further that all cultures currencies utilize metal that is roughly as pure as the other. Historically, that was never true, as debasement of the currency was common, but its a convenient assumption.
One of the biggest problems you are going to run into is that 3e still contains traces of the 1e duel economy that has plagued the game since its inception. On the one hand, Gygax was a pretty decent amateur historian and priced most common goods fairly accurately based on a silver piece economy that is described in several places in his rules (taxes paid by serfs, wages earned by laborers). On the other hand, Gygax had a gold piece based economy that the PC's operated in and by which everything commonly used or needed was priced. Within the gold piece economy, nothing is priced according to its actual economic cost, but rather according to its assumed gamist value (as for balance purposes). Gygax justifies this duel economy in the 1e player's handbook, explaining that the default prices assume the PC's are operating in an area of hyperinflation similar to that of the Klondike during the gold rush and as such all adventuring equipment is proportionately overpriced. Unfortunately, almost no one has ever paid attention to that claim and took it seriously, and certainly not later authors of price guides. In fact, I've seen several published modules were equipment was said to cost double or even triple the prices in the Player's Handbook (think about it).
This usually produces no real problems until you try to convert between the two economies. The result gives the Player's truly enormous buying power if they choose to leverage the silver piece based labor economy, probably far exceeding the difference in buying power between first world and third world currencies. The PC's can use this fact to 'go Belloch', buying up enough labor to dig up dungeons rather than traversing them, or to by micromanaging convert labor and raw goods (priced in silver pieces) to finished goods (priced in gold pieces) and turn higher profits than they could by exploring (if the DM isn't up to full blown economic simulation).
In my own game, I solve this problem by converting all economic systems to a 'realistic' silver piece based one. Silver is as valuable in my game as gold is in most games. A few gold pieces is treated as quite significant wealth by my 7th level PC's. A simpler but equally effective solution is to convert the economy of the peasants to a gold based one and assume 1 gold piece per day is roughly the socially acceptable minimum wage. That works fine two.
In general, as a rough guide I assume that 1 s.p. = $50 of buying power. Thus, at a 20:1 ratio, 1 g.p. = $1000 of buying power. This tends to work pretty well provided you remember that mass produced goods aren't available in the society and thus all goods are priced at roughly the price you'd see for custom made hand made goods. As such, when I can't find an historical source, thinking about things in this way let's me ballpark a price for just about anything. If you convert to a g.p. based economy, then just set 1 g.p. = $50, with 1 s.p. being $5 or $2.50 depending on whether you use a 10:1 or 20:1 ratio.
You may eventually see from that why I prefer a more historical silver based economy. While you could of course assume gold and silver are of sufficient abundance in your world that they have much lower value, one problem you'll quickly run into is one I ran into way back in my early days as a DM 30 years ago - the weight of anything of significant value is enormous. D&D assumes 1 coin is roughly 1/10th of a pound, but even if you assume that a coin is 1/16th of a pound or 1/20th of a pound, you'll quickly find that to pay for anything with real value requires tons of metal. If gold is a mere $800 per pound, then buying a mansion with the stuff (say 2.4 million dollars of real property) requires 3000lbs of gold. Global economies can barely function with such a debased currency IMO. The cost and hassle of transport is just too great. Almost certainly merchants would convert to a practical paper economy out of necessity, and nations would likely need to create much more valuable coinage in platinum, mithril, and adamantium just to cover the scale of their expenditures.
