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Regarding the prices in castles, here's what I'll say: in all cities, the total cost of units is 28,000. This is the amount of money that comes from one capitol per week, so you can only buy an army in 3 fully built cities if you have 4 mines.
And of course, the main question is: why buy all the units? Buy only the best units, or those on which your strategy is based.
In the worst case, play as humans. In Elaine's case (at least in the original game), the gold income from peasants is increased by 2 times. With 3 fully built castles, you get about 150 peasants, which is 300 gold per day.