I apologise for a mistake i made. I should have written PQ > 30+ then 20+.
100 Billion Pop planet Gives you around 1500+ gc by itself(class 38 planet). Basically once you have one you dont even care about the cash anymore. I never said you can turn any planets into 100 Billion ones (class 35+ makes so good ones

). I do agree however that if you can not build a 100 Billion planet it is better to go with 5 (Then 12) billion planets. The difference between 100 Billion and 12 Bilion is minor (aound 30% with 5B and 10% with 12B) in my last game and around 200%(cos i had two of them out of my 12 planets

)). Also a factor to consider is the amount of morale you have. In my game (I have 1 moral ressource) and 62% tax. If i scrap my 100B planet and increase my tax to 70(so the morals are pretty much the same). I loose in total around 1000 in revenue (ie transforming a 100B in 12B and changing farms to Stock exchanges).
Obviously if your 12B planets tax is 50% forget it.
In fact i have made some research about it and here is the approximate economy formula for each planet (approximate cos there is something like 5% variations):
Square Root (Population) Tax Rate * (100% + Buildings Bonus) * (100% + Racial Bonus) * (100% + Gov Bonus)
This means that once you have reached 45% percent tax with you 5B planets you should go 12B. 12B planets gices you 56% more income and cost 30% approval. The 5B cost 9% approval. 30% tax (which you would set for 12B planets) cost 33% approval, while 45% tax (which cost 54% approval). The net result is 0. Since the tax increases faster from 45% then from 33%, it is better to go on with 12B.
Getting more then 12B (ie build more then 1 FARM) is economically inefficient.