There are 3 types of "goods":
- Food
- Product
- Oil
The average cost of food is $20.
The average cost of product is $40.
The average cost of oil is $110.
If you buy lower than average, you will usually be able to find a country to sell to that is at or above that price!
It is always a good idea to check first, though - if the "goods" you are trying to sell are down in the world market, you'll be taking losses.