The prices of resources are not fixed, but are based on player-driven market activity. If more cities are selling than buying, then the prices of goods will drop. Conversely, the price will rise if the buyers outnumber the sellers.
The Global Market is optional. If you want to isolate your city from the rest of the world then you don’t need to place a Trade Depot. However, keep in mind that resources are limited. If your electricity is being generated by coal or oil then you will need to shift to renewable energy sources when you exhaust your regional supply.
For the entire blog go here (needs an origin account): THE BUSINESS OF THE GLOBAL MARKET