So Civ 5 has an invisible feature where any unit in the game costs 1 gold to maintain (This includes settelers and builders). the cost to maintain any unit increases as you evolve into different eras... Long story short make sure you carefully observe: how many units you have, how much gold you produce, how much buidings cost to maintain, and other such expenses. Gold is usefull for instantly building things as well as being an important feature in citizen happiness.