So digging through the chronicles of history...
Here was my original design around AI valuing non-PU resources (along with a bunch of related posts from @Frostion and myself): http://tripleadev.1671093.n2.nabble.com/AI-Development-Discussion-tp7585227p7590752.html
Here is the initial design:
Find resource that I have the most of (A).
Find unit cost in terms of A by converting all other resources (B, C, etc) to this resource using ratio of this resource to the most resource.
Use this value instead of the PU value that is used today for finding unit efficiencies.
Example #1:
Current resources and amounts:
A - 50
B - 5
Units:
U1 - cost 1 B, very weak unit
U2 - cost 1 B and 10 A, stronger unit (3x 'power' of U1)
#1 purchase (50 A, 5 B | 1B is worth 10A)
U1 would be evaluated to cost 10A with efficiency (power/cost) of 1/10
U2 would be evaluated to cost 20A with efficiency of 3/20 (purchase this unit)
#2 purchase (40 A, 4 B | 1B is worth 10A)
U1 would be evaluated to cost 10A with efficiency of 1/10
U2 would be evaluated to cost 20A with efficiency of 3/20 (purchase this unit)
And so on, so the AI would most likely buy 5 U2's which is good in this case.
Example #2:
The AI’s bank has 52 PUs (A), 26 Manpower (B) and 13 Coal (C)
Infantry: 6 PUs/5 Manpower, power~7
Artillery: 8 PUs/2 Manpower/1 Coal, power~8
Tank: 10 PUs/1 Manpower/2 Coal, power~9
(power values are just approximate showing that units are in decreasing efficiency based on just PUs, tend to buy most inf then art then tank ignoring new resources)
#1 purchase (52 A, 26 B, 13 C | 1B is worth 2A, 1C is worth 4A)
Infantry would be evaluated to cost 16A with efficiency (power/cost) of 7/16
Artillery would be evaluated to cost 16A with efficiency of 8/16 (purchase this unit)
Tank would be evaluated to cost 20A with efficiency of 9/20
#2 purchase (44 A, 24 B, 12 C | 1B is worth 1.83A, 1C is worth 3.67A)
Infantry would be evaluated to cost 15.15A with efficiency (power/cost) of .46
Artillery would be evaluated to cost 15.33A with efficiency of .52 (purchase this unit)
Tank would be evaluated to cost 19.17A with efficiency of .47
.. 3 more artillery, I believe
#6 purchase (12 A, 16 B, 8 C | 1B is worth 0.75A, 1C is worth 1.5A)
Infantry would be evaluated to cost 9.75A with efficiency (power/cost) of .72
Artillery would be evaluated to cost 11A with efficiency of .73 (purchase this unit)
Tank would be evaluated to cost 13.75A with efficiency of .65
So it would end up buying 6 Artillery with remaining 4 A, 14 B, and 7 C. Though each time it will value B and C less since A is becoming worth more as it is spent too quickly as you can see in purchase #2. This makes infantry become more and more efficient in comparison to artillery since it has the lowest PU cost. The AI essentially tries to spend resources as evenly as possible but the unit set is weighted too much towards costing PUs in comparison to starting resources.
Follow up design (adding demand based):
The algorithm has relatively few steps based around resource supply but has lots of power due to the feedback loop in it. The feedback loop allows the AI to adjust valuation of resources as they are spent so at first a very sparse resource will be worth a ton but as the AI spends its other resources the very sparse resource value becomes less and less.
You bring up a good point about the available units. This algorithm so far is only a supply side algorithm as it only evaluates what/how many resources you have. To take the algorithm to the next level (more complex) you would then add in 'demand' ratios as well (how much of the resource is in the purchase options).
So if you had a very sparse resource (like B above) and it is needed for every unit available for purchase then B is really worth even more (you pretty much don't care about anything but B since its the limiting factor). The opposite is if you have say 100 units to choose from but only 1 cost some B then B isn't worth quite as much since you have lots of other options that don't need B.
Taking the above example with 1 unit that cost 10A and another unit (equal strength) that cost 1B. The average A cost across the unit set would be (10 + 0) / 2 = 5A per unit and B would be (0 + 1) / 2 = 0.5B per unit:
50A / 5A = 10 units worth, 1B / 0.5B = 2 units worth, so unit#2 cost 5 times unit#1 (50A) - no change to the above
Unit set skewed towards A: Now let's say we have 9 units that cost 10A and 1 unit that costs 1B. The average A cost across the unit set would be (109 + 0) / 10 = 9A per unit and B would be (09 + 1) / 10 = 0.1B per unit:
50A / 9A = 5.56 units worth, 1B / 0.1B = 10 units worth, so unit#2 cost 5.56/10 (0.56) times unit#1 (5.56A) - would definitely purchase unit#2 first
Unit set skewed towards B: Now let's say we have 1 unit that cost 10A and 9 units that costs 1B. The average A cost across the unit set would be (10 + 09) / 10 = 1A per unit and B would be (0 + 19) / 10 = 0.9B per unit:
50A / 1A = 50 units worth, 1B / 0.9B = 1.11 units worth, so unit#2 cost 50/1.11 (45.05) times unit#1 (450.5A) - would definitely purchase unit#1 first
You can see by adding this 'demand' ratio this then checks to see how much of the various resources are needed across the available unit set to take into account whether the unit set is 'balanced' across the various resources. It then adds more weight to resources that are needed the most in the unit set.
