There should be several levels of infrastructure, each based on :
- number of population
- number of PP (or whatever ecomic meter is relevant) put into developing infra levels (although I think that like in MOO or Stars!, pop should produce some small amount of PPs itself and be required to 'work' in inf to produce more PPs)
- tech level of the empire (some levels require advanced tech)
In shorth, think of factories in MOO or Stars!, but without 'hundreds of factories per planet' but rather a single number for planet (example: planet Blah: infr level 4, devoting 44% of PP to improving inf level, ETA ceterus paribus for level 5: 68 turns).
Inf growth can be speed up by ferrying population/resources to a given planet, but wiithout some very advanced tech, it will always take TIME (as in many, many turns). Example: planet BlahPlus: infr level 1, devoting 100% of PP to impr inf level, ETA for level 2: 100 turns. Population: 0.1% max, transports on route ETA 3 turns: equivalent of 1% of planet max + some resources, ETA after their arrival: 25 turns)
What is known as buildings, or wonders (small, big or otherwise), or just various tech inventions concerning infrastructure should have minimum inf level requirement (besides some others based on focus or other stuff relevant to specific wonder/tech). Many inventions should be applied without player intervention to any planet that can benefit from them. So to benefit from, lets say, holographic movie theaters planet should have a inf level of 3 or greater - and once a new colony reaches the level 3, or player invents the tech, it gets the benefit without requiring player to click a single button on UI.
Infrastructure should be the basic thing that generates other PP (or blah relevant blah meters blah stuff like food, science etc.) , of course modified by other stuff like planet type, techs, wonders, blah, blah. Infrastructure requires people to work on it.
Now lets see how it works in practice watching a colony developing through several infr leves (I am leaving tech/wonders out).
1. New colony (cannibalisng colony ship, building self-sustaining colony - one minor city with several outlying villages)
cost: no (special - created from colony ship)
pop min: no (ditto)
pop max: 10% of planet max pop (low tech base makes bigger population impossible to sustain)
PP from population multiplier: none (1)
Some numbers off my head based on Stars!
World max pop: 1,000,000, pop growth per turn 10%
Pop carried by colony ship: 2,500
Each pop can produce 1 resource per turn
Let's assume that without influx of more colonists/empire it should take 50 years/turns to reach each next level.
2. Low tech colony (with few adv tech artifacts from colony, build XIX century economy based on local resources, several cities)
cost: 100,000 PP
pop min: 10,000
pop max: 50% of planet max pop
PP from pop multiplier: 1.5
Note that when population on planet drops below pop min, it is assumed that people left on planet cannot sustain the infrastructure on given level and we go down with PP multipliers - think of not enough people to work in power plants, mines, universities, etc.)
3. Med tech colony (around XX century tech, dozens of cities covering perhaps one tenth of planets habitable)
pop min: 50,000
pop max: 100% of planet max pop
PP mult: 2
4. Adv tech colony (XX century tech, similar to year 0 HW - allows space flight techs, hundreds of cities, several megalopolises, spanning on at least half of planetary surface)
pop min: 250,000
pop max: ditto
PP multi: 3
5. High tech colony (adv tech, most of planet surface inhabited)
cost: 100,000,000, blah, blah
6. City planet (adv+ tech, planet covered with megalopolises, 100% of planetary sufrace inahbited, oceans dried up and inhabited, subterrean and orbital habitats) blah blah
This is just an example, not too good at that - population growth and related occupation of planetary surface is an important factor that I just glided over here (I would like to go with more levels and severals trees of growth depending on ratio of population to PP invested in infra), but I hope it shows the basic ideas I have for infra.