I'm here to just throw my ideas about the windmills and how it should work.
About material cost and individual energy production:
First, windmills should have 3 tiers, Basic, Advanced (like the induction furnace) and Industrial.
Tier 1 windmills are made with 1 basic machine casing, 2 iron shafts and 6 iron plates.
Tier 2 windmills are made with 1 advanced machine casing, 2 steel shafts and 6 dense steel plates.
Tier 3 windmills are made with 1 tier 2 windmill, 2 advanced alloy shafts and 6 (reinforced) iridium plates.
Rotors efficiency are as follows:
Advanced Iridium Alloy: 50% (Adv Ir Alloy rotors last forever and does not wear down, but its weight reduces its efficiency)
Rotors, those should have variable sizes, for people's different needs (and avoid excessive spam of windmills for high power systems) :
Small Rotors are made with 4 blades and iron fence. These all occupy a space of a 3x3 square in the front of the windmill.
Small blades are made with 6 material + "glue" material
Wood: 6 plank and 3 sticky resin.
Iron: 6 iron plates and 3 iron fences.
Steel 6 steel plates and 3 iron fences
Carbon: 6 carbon plates and 3 steel plates.
Advanced Iridium Alloy: 6 advanced alloy plates, 2 carbon plates and 1 iridium plate.
Medium rotors are made with 4 medium blades and iron shaft. These can only be used on the tier 2 windmill. These occupy a 9x9 area.
Medium blades are made with 6 small medium blades.
Large rotors are made with 4 large blades and steel shaft. These can only be used on the tier 3 windmill. These occupy a 27x27 area.
Large blades are made with 6 medium blades.
Average Energy production at most efficient height using small rotors:
25 kU/t * rotor efficiency.
Average Energy production at most efficient height using medium rotors:
175 kU/t * rotor efficiency.
Average Energy production at most efficient height using large rotors:
1225 kU/t * rotor efficiency.
Now the obstruction windmill mechanics:
Windmills should not stop working if another windmill overlap its area (the ~32 block detection in the perpendicular axis, overlapping rotors of course should stop working), but instead reduce both windmills power production using this formula:
Energy generated = Overall (from the rotors n such above) * 0.995^X * 0.95^Y
X = number of obstructions of any kind other than windmills, Y = number of other windmills partially obstructing the wind.