All trade ports get the same amount based on the longest possible path between two of them (measured in jumps)Not entirely true, any trade port not connected to the main "chain"(the big white line you see when hovering over credits amount) only gets an income based on the longest distance between it and the end of what I call one of the "misfit chains" But all trade ports on the same chain will generate the same amount of income each.
Not quite...
Trade port income is based on your longest unbroken chain of planets with trade ports (exceptions: unconquerable "planets" - i.e. stars, space junk, asteroid fields, etc.). All trade ports will earn this income, whether or not they're in the longest chain. Building more trade ports on a single planet will increase the income earned from that planet (methinks linearly, but I've never done the math for sure), but will not affect the rate of all the other trade ports on other planets, regardless if they're in the chain or not.
The only ways a trade port can earn more than any other trade port is if it's next to a planet owned by a player with which you have a trade alliance, or if it's on a planet that has some sort of bonus to trade income.
If there's something I've missed, somebody correct me.