How are the percentage and colors for item prices calculated?

I don't get how the percentage for items is calculated.

Example:
I buy 1x "navigation data" for 1'199 units (the percentage appears in green color "-27,3%").

The next time I want to sell it (at a terminal or to a NPC) I get 1'003 units as price (the percentage appears in red color "-39,2%").

Who or what is calculating the base price/percentage for an item and how?

Solution 1:

As I understand it they are governed by the star systems economy. Different star systems produce different types of goods. There are seven types of economy and every star system has one.

If you have an Economy scanner built into your ship you can see the economy type and prosperity from the Galaxy Map.

Items which are in high demand in the star system will be purchased from players at a higher price and be marked with a green +Example% showing how much the market will pay over the regular price, and obviously the red -Example% if the item is abundant.

Knowing this leads you to wanting to setup Trade Routes between star systems which produce goods which are in demand in nearby star systems.

As mentioned on the wiki

Resources replenish slowly and the fulfilled demand at a target lowers the price for consecutive sales

So it's worth noting that if you sell more items, the price you get will decline.

The wiki also notes that it's worth having more than one trade route available to allow demand to recover.

An easy bridge to remember the two trade routes are the terms ITEM and CAR. Industrial-Technological-Energy-Mine and Commercial-Alchemy-Research.

These are the star system economies you'd want to visit in turn to maximise your profits.