The production and economy system in GC2 was pretty counter-intuitive to me also at first, based on my experience with other games of this type. Here's a basic overview:
- Income is purely population/tax based. More population means more income, at a given tax level.
- Market/trade buildings give a stackable % bonus to the planet's income, so more buildings means a bigger % bonus. *But*, if you build them on a planet with a small population, there won't be much base income, and so even a huge bonus won't amount to much actual income, since it is percentage based.
- So, to raise income, increase population. Population grows more quickly with higher approval and when planet approval is > 75%, it gets a bonus. And at 100% approval growth is doubled.
- Since raising taxes hurts your approval, it also hurts your pop growth and so too your long-term income. Higher planet populations also make your approval ratings drop on those planets.
- Since approval is important for pop growth, morale-boosters are important to keep morale up and your pop growing.
- You also need to build farms to raise the planet population cap, but be carefult to also budget space on the planet for enough morale boosters to keep the pop happy. I've found that beyond 20 billion planet pop you'll need 3+ morale boosters.
- Factories have nothing to do with income; they only let you spend your money faster. Building more/bigger factories are just increasing your production capacity. But of course, that's a critical thing as many others have shown above. Just keep in mind that if you're strapped for cash, building more factories will only increase your spending rate not your income.
- Each factory production point costs 1bc, whether for military or social production. As of one of the patches (1.x?), unused social production is diverted to military production. Unused military production is "free" - it is not wasted and you don't pay for it, but it also doesn't generate income.
- I've found that the later building upgrades can be painfully slow if your planet has low production capability, so I like to have several factories per planet (assuming a size 10+ planet) to help fill up and upgrade all the planet slots. Then, once the planet is full and fairly upgraded, so can upgrade some of all the factories to other building types.
- Economy starbases are incredibly useful because you can upgrade them to give up to 25% bonus to income, production and research. Half the bonus production and research is *free*, the other half you pay for. So if your planet has a base 100 production and 100 research, a +25% starbase will give you 125 production and 125 research, but you'll only pay for 113 of each.
- The starbase bonuses are stackable, so if you can build 4 full economy bases covering your worlds, you're literally doubling their output.
Hope this helps.