Diplomacy, Trade and Spies

In this section

How do I establish trade? How can I earn money?
Standard prices
What are the standard prices for goods?
City favor
What are the different favor levels of other cities? When will a city agree to trade/form an alliance with me?
Gifts & Requests
What can I request from other cities? What happens when I give goods to other cities?
How can I use spies effectively? What damage can other spies do to my city?