yes. i believe you get a flat 1.3 /minute for every trade post, period regardless of how many other posts are in that grave well. You get more by having long trade routes. I don't think having multiple posts in the grav well boost the size of the trade route though.
And if the devs were smart, it should be much more efficient to build a post that lengthens your trade route than build another post in a grav well that already has one. Like 2 or 3 times more efficient.
1.3 credits per minute? are u sure.. that seem very slow, given the investment required.. if its a credit rate similar to the civilian tax, shouldnt it be by the secound?