The planet type and what you intend to do with colonies greatly influence how much money you can make from them.
For example, my first colony was on a barren planet, making biofuels. I was only making like 10k a day from it. Although I was only using a lvl1 command center. IF I had trained up a bit more and bought something like an elite command center, I could refine my produce into a finished product right there on the planet, selling it for 10x as much and selling 10x more per trip. Probably making a good 500k per day from doing absolutely nothing. After a week the colony would then pay for itself, if you did things right.