I've seen a lot of threads with complicated discussion of economic math and formulas, but very few that actually discuss what you need to do to get a high income. As such, here is a fairly successful strategy that I use, that hopefully some other people can tell me how to improve.
Basically, any planet that doesn't have either two or more +100% tiles or 1 300% tile for research or production becomes an economy planet. That is, for PQ 10 or greater, I build two farms, two entertainment centers, and all economy buildings. Maybe a cultural exchange center if there's a tile for it. For less than PQ 10, one farm and one entertainment until I see how many tiles I get through the terraforming techs. Then, once I've researched soil improvement, I'll put a factory on each of these economy worlds to help with the construction of the more advanced projects. Late game, each of these planets can produce 500 BC a piece.
The first PQ 17< planets I colonize usually are geared towards heavy research and production, but later on, I'll start using them as economy planets, and these can really produce a good deal of money. 3 farms, 3 entertainment buildings, 1 factory, and the rest economy structures. The highest I've gotten the income from these is 1500 BC, but some people have gotten close to 10,000. (I don't know how though, with a class 32 planet, 20bil, and a 69% tax rate, the most I've ever gotten is 1500bc.) These high class planets are usually research, but if there are no bonus tiles, I'll sometimes make them economy planets.
Late game, my economy approaches 20,000 bc a turn with a huge map and habitable planets set to common. Economy resources can jack this up even higher. With abundant planets, I'd imagine this can get even higher.
For racial bonuses, I play as a custom race, with Economy +30%, and Morale +15%. With planetary populations of about 16bil late game and all the morale improvements researched, my approval stays at about 97%, even with 69% tax rate. I suppose if you're brave, you can drive the taxation rate up even higher, or research higher end farming techs, but I never go past the second level. I'm always sure to buy the morale improving trade goods too, if an AI happens to get them first, trade for them. The rest of my points I sink into research and weaponry/defense/hp.
This is good, because my most expensive late game ship, a huge hull with 350 attack and 200 defense, costs 45,000BC to buy, and 108BC a turn to maintain. A lot of these can put a pretty big dent in your economy, if I were to build 50, I'd lose almost 5000BC a turn in maintenance. (I like this though, I think ridiculously powerful ships should have some sort of limiting factor.)
