According to this tech tree:
http://davemcw.com/factorio/tech-tree/
Mining productivity 1 - 100 red, 100 green = 750 Iron, 250 Copper
Mining productivity 2 - 200 red, 200 green = 1500 Iron, 500 Copper
Mining productivity 3 - 300 red, 300 green = 2250 Iron, 750 Copper
Each level yields a 0.02 improvement. So to break even on my 750 investment back I need to mine another 37500 Iron (2% of 37500 = 750).
This part is wrong.
So the first level calculation is correct. The second one is where the trouble starts. Say you have 4500 iron and you buy all three levels at once (for example). How much ore do you need to mine to get your 4500 back?
For the first 37500 each of the three productivity bonuses has to pay itself off, but they all do so simultaneously.
Code: Select all
After 37500 ore mined
-----------------------------------
Cost - Return = Remnant debt
1: 750 - 750 = 0
2: 1500 - 750 = 750
3: 2250 - 750 = 1500
Code: Select all
After 37500+18750 ore mined
-----------------------------------
Cost - Return = Remnant debt
1: 750 - 750 = 0 <-- This one brought in a profit of 375 applied below
2: 1500 - 750 - 375 - 375(1) = 0
3: 2250 - 750 - 375 = 1125
Code: Select all
After 37500+18750+18750 ore mined
-----------------------------------
Cost - Return = Remnant debt
1: 750 - 750 = 0 <-- Profit of 375 applied to (3)
2: 1500 - 750 - 375 - 375(1) = 0 <-- Profit of 375 applied to (3)
3: 2250 - 750 - 375 - 375 - 375(1) - 375(2)
4: 16600 - 750 - 375 - 375 = 15100
Note: Assumes smelters, assembly machines and labs needed to make and consume the science packs are all free. So the iron and copper costs are a lower bound.
Edited after Aeternus' comment which lead to a math review. Unfortunately Mavzor's graph is wrong.