The prices are the same for everyone in the world. But as GC13 said, you get to pick the goods that you produce first, and remaining is sold off to the world market. Prices rise and fall all the time depending upon the supply and demand, so for example when in mid-game countries begin to industrialize there is a lot of demand for steel and it sells expensive.
Not sure how it works if you are in a sphere. I guess the goods you produce are sent to the common market, where every sphere member deposits his goods and sphere master (i.e. the great power) buys whatever it needs first, and the members buy the what remains in order of their prestige (or rank?). After every sphere member has bought what they needed, the leftover is sold off to the world market. Or perhaps here you get first picks on goods produced in your nation as well, before sphere market?