Planetary Wealth is an approval bonus/penalty. You can see it if you hover over Approval in the planetary screen. I intended to reverse engineer it, unsuccessfully, but I found some things that are a step in the right direction.
1. Planetary Wealth is a function of Wealth and Maintenance. As far as I can tell, there is no other variable.
2. The reason changes to the High Approval production bonus do not apply this turn is because if it did it would impact Wealth, which would impact Planetary Wealth, which would impact Approval, which would impact the High Approval production bonus. The game cannot allow this kind of circular math.
3. In all my tests, having Wealth = Maintenance + 2bc, the Planetary Wealth is 0%
4. The Planetary Wealth function shape resembles an exponential curve, i.e. Planetary Wealth increase/decreases faster than linearly as Wealth - Maintenance increases.
5. Wealth improvements/research/starbase modules are also approval improvements, assuming some production is allocated to wealth. So are farms, eventually.
Okay Gilmoy, you're up. What's the function?