Okay, time for another long post. This is on balancing the upgrade times and costs of improvements. It was pointed out that Shaft Mine takes no time to upgrade to Modern Mine. This is a problem. Some other upgrades either don't have costs or have costs that are highly out of line with other upgrades at the same time. I have done some calculations towards balancing this.
I went and pulled out the upgrade times and costs for all improvements that can upgrade.
Improvement Upgrades To Yield Time (base) Gold (base)
Mine Shaft Mine +1P, +1C, -0.5H 40 25
Shaft Mine Modern Mine +1P 0 40
Workshop Industry +1P, +1C, -1H 26 50
Industry Industrial Complex +2P, -1H 36 can't
Woodcutter Lumbermill river only: +1P 10 0
Windmill Windfarm +1C 10 50
Windfarm Windtrap +1C 20 10
Cottage Hamlet +1C 10 10
Hamlet Village +3C, -0.5H 20 25
Village Town +1P, +1C, -0.5H 40 50
Jungle Camp Forest Preserve +2C, +0.5H 20 0
Forest Preserve Treefarm* +2P, +1C 32 0
Treefarm Hybrid Forest +1F, +0.5H 20 0
Treefarm is marked with an asterisk because Forest Preserve currently upgrades directly to Hybrid Forest. This is a mistake and will be fixed.
Overall, I am assuming that the Cottage line of upgrade times is balanced. It should take more time to upgrade from a stage-2 improvement to a stage-3 than from a stage-1 to a stage-2 because the number of citizens it takes to work a tile doesn't change. You get more improvement per citizen at the end.
Also, I thought about how to balance the costs to upgrade improvements. What I settled on was this: the cost to upgrade an improvement should balance out with the bonus yields for upgrading the improvement right away. The total yield for an improvement is calculated as food + hammers + 1/2 commerce (remember, I count 2 commerce = 1 food) + 1/2 health change. Production costs 3 gold/hammer to rush. The base upgrade time is modified by game speed; it's 1.6 on Normal speed (Cottage to Hamlet has a base time of 10 and takes 16 turns on Normal speed). Finally, I put in an equalizing factor of 0.5 because you don't get all the bonus yield at once. If you waited the full upgrade time, you would have an equalizing factor of 0. I think 0.5 is fair.
The trickiest part is that I also multiplied the gold cost by 2 for each previous upgrade. I think this covers the increased efficiency per citizen part of the upgrades.
Cost to upgrade an improvement: yield change * 3
[3 gold/point of yield] * base time to upgrade * 1.6
[time to upgrade on Normal] * 0.5
[equalizing factor] * (2^number of previous upgrades)
covers increased efficiency per citizen
So here is my corrected table of upgrades.
Improvement Upgrades To Yield Time (base) Gold (base) Gold (calc) Gold (rounded)
Mine Shaft Mine +1P, +1C, -0.5H 20 25 60 60
Shaft Mine Modern Mine +1P 40 40 192 200
Workshop Industry +1P, +1C, -1H 20 20 48 50
Industry Industrial Complex +2P, -1H 40 50 288 300
Woodcutter Lumbermill river only: +1P 10 5 12 10
Windmill Windfarm +1C 10 10 12 10
Windfarm Windtrap +1C 20 25 48 50
Cottage Hamlet +1C 10 10 12 10
Hamlet Village +3C, -0.5H 20 25 120 120
Village Town +1P, +1C, -0.5H 40 50 480 480
Jungle Camp Forest Preserve +2C, +0.5H 20 30 60 60
Forest Preserve Treefarm* +2P, +1C 40 40 480 480
Treefarm Hybrid Forest +1F, +0.5H 60 50 720 720
Does this look good? I think some of the end game numbers might be too high, but there is usually a lot of gold being thrown around at that time.