As an aside, do wonders seem fundamentally incapable of being balanced?
Things like the London exchange (4320) basically trades a stack of musketeers for 500-1000 gold per turn before coefs and marketplaces and such are added. This is basically a full renaissance age advance per turn. This adds another modern age advance once every ten turns effectively. This happens about 3x's as quickly with the theocracy gold coefficient and city improvements included. Is there any amount of cost that makes this reasonable. Maybe the trick would be to convert this into caravans so like 4 caravans bring 4 new goods into each city so (50+40+30+20 (minus the ten you lose in each city it comes from) so 40+30+20+10 = 100 gold) This gold is before the gold coef of the government is applied just like the free gold from maintainance. So we're looking at 20 to 40 caravans equivalent. This means a production cost of about 15,000 to 30,000. This may work out to be really similar because the disadvantage of the caravans is that they require goods to use. The advantage is that they never expire (though they can be pirated) and as they're being built they can be used. So once 7,500 production is pushed into caravans those first ten are out and being used, while the wonder is still building.