Trade prices are affected by your relations with the country you are offering a trade to / receiving a trade from. The prices are the same for both buying and selling items.
The base prices are ($/unit):
|Trade Good||Base Price|
In terms of multiplier value:
Relations will reduce/increase the cost. The equation for final cost is:
Price = Base Price * ( 1 - Relations * .00165)
Trade agreements increase relations with foreign countries by 15 points each. The volume traded does not affect the relations increase. Increasing relations with a number of small, cheap trades prior to requesting a very large trade can save a lot of money.
Also of note, it is almost always a good idea to buy energy as opposed to fuel or crude oil as the cost is dramatically lower and a little bit of research into conversion techs is usually more than enough to supply most nations needs, the exception would be a nation with a primarily MOT or ARM force which has a significantly higher fuel requirement.
How to Trade
Click on diplomacy. Sort countries by resource. Currently the default sort is to show the least amount available. Re-sort again. Now you will see white icons. Red icons are zero availability. Offer trade for small amount. In the beginning you have to trade with a major power that has money available. This may be your opposite faction. Pump up the world market by resources. Once a country has some cash flow from you they will accept a trade to buy your resources.
Notes on Autotrade
Autotrade has two significant advantages and two disadvantages.
+Autotrade can set up a trade for more than the 50 units of resource that a player can manually.
+Autotrade will also attempt to acquire as close to the exact amount of resources needed, thus being more efficient than a human player.
-Autotrade does not export, which can cause a large deficit in money which, by the time noticed, cannot be counteracted after all AI nations are content with current trade.
-Autotrade will trade with any faction, which can impact the player's nation's factional drift in undesired ways. This can be fixed by canceling trade routes with unintended nations, causing autotrade to search for resources from different nations.
A Note on Supplies
It is worth trading for resources to increase the amount of IC you can fund, even if you have to sell more supplies to earn the money (or even put IC to Consumer Goods). At base values, it costs 0.3 Money to fund 1 point of IC; however, 1 IC will produce 0.05 money passively, plus whatever money you can get from producing supplies. Even selling two units of Supplies per IC will be easily enough to cover costs, allowing you to put the rest of your IC to useful work.