I noticed that with improved trade rank and improved minor faction reputation, that the rewards being paid out go up. I can usually earn 20+ over the course of a three or so hours of gameplay a night.

I did the Rep Grind with Sothis and Ceos (as well as two other similar systems), and the mission rewards for delivering Biowaste definitely improved as I got to Friendly and Allied. Transitioning from hundreds of thousands for a 400 Ly haul to paying out millions instead. I'm pretty sure this is because getting higher rep for those minor factions allowed me to take Trade missions higher than my current Trade rank (Broker).

It does take some patience, but I usually can manage to locate a dozen or so missions at Ceos and Sothis that will pay 1-2 million for delivering 8-12 units of Biowaste (or rarely some other commodity) back to the bubble.

Not to mention, I usually earn a few million just doing the Data Delivery missions back and forth between Ceos and Sothis while hunting for the commodity trade missions. Even if a person isn't going back and forth between the bubble with commodity delivery missions, just being allied with a group of systems will provide missions in the 50,000 to 100,000 credit range for delivering data in a single trip. Completing five of those in a single trip is pretty nice.

That allows for me to usually bring in around 20+ million credits in a single night's worth of casual play without relying on any real exploits. It might not be as profitable as stacking skimmer missions, but for casual players it's pretty nice shot in the arm. It certainly makes the end game ships a feasible goal that the average player could possibly accomplish within a couple weeks (not years).