There are three factors that affect the value of the trade route: 1) its length, 2) its age, and 3) the wealth of the planets it connects.
Because of (3) and because it usually takes me a while to explore, I tend to send freighters to the other civ's homeworlds, which tend to be the richest. I notice the AI consistently sends its freighters to the most distant worlds I have, to maximize (1).
