In Spore's Civilization stage and Space stage, your civilization or empire can trade with other nations. This builds up an economy, which governs your nation's treasury. It depends on how many Spice geysers your nation or empire owns and how many trade routes have been iniated producing your nations income, and also how many factories exist in the various colonies or cities. Slavery or other types of mining are not ways that your nation or empire can improve their economy, though Modifications can change that.
Through Trade, Relation Bonus, and other types of income, you can start a trade route with another star system or city and eventually buy their city of star system after waiting. But, if you bid for the star system too low, it could insult the empire, making them not let you purchase the system.