I don't know the routes too much but what I've seen so far was all equally well distributed among the systems I've visited. Which isn't much, but anyway. I also try to stick with my RP which is a Liberty-based trade corporation (Universal) so obviously I'm hauling stuff to and from Liberty, that gives me a lot of different trade routes to choose from and (quite likely) restricts me from doing the really profitable ones. How much should a good trade route net you?