An example on relative values. Say Civ A wants to buy horses from Civ B. Civ A has 4 horses and produces 21 gold per turn. Civ B has 12 horses and produces 14 gold per turn. Case 1 horse. Civ A values 1 horse as 1 on 5 horses it will end up with. Since horses is the only strategic resource this far, it means increasing strategics by 20%. Civ A might be willing to pay a maximum of 20% of its gold production, so it will pay no more than 4.2 gold per turn for 1 horse. But since the horse is wanted for a horsemen unit, then the cost of maintaining that soon to be unit has to be included. Say it will cost 1 gpt in maintenance. Then Civ A will pay a maximum of 20% of its spare gold production (20 - 1) * 20%, so 4 gpt. Civ B values 1 horse as 1 on 12 horses it has connected. Selling one horse means reducing strategics by 8.3%. Civ B might be wanting to receive a minimum of 1.26 for that horse. Theoretically, a fair price would be the average, 2.63 gold per turn for 1 horse. Case 2 horses Civ A values 2 horses as 2 on 6, so a 33% increase. 2 horsemen will take 2 gpt from budget. Civ A will pay no more than 6.3 gold per turn. Civ B values 2 horses on 12 horses it has, as 16% of strategics. Civ B wants a minimum of 2.78 gold per turn. x / (14 + x) = 0.166 <==> x = 0.166 * (14 + x) <==> (1 - 0.166) * x = 14 * 0.166 <==> x = 14 * 0.166 / (1 - 0.166) = 2.78 The average is 4.54 gold per turn for 2 horses, so every horse costs 2.27 gpt. Case 3 horses Civ A values 3 horses as 3 on 7, so 42.8%. Say 3 horsemen cost 3 gpt in maintenance. Civ A will pay as much as 7.7 gpt. Civ B values 3 horses on 12 as 25% Civ B wants a minimum of 4.6 gpt Average is 6.15 gpt for 3 horses, so every horse costs 2.05 gpt. Case 4 horses Civ A values 4 horses as 4 on 8, so 50%. 4 horsemen is 4 gpt in maintenance, so 17 spare gold per turn. Civ A will pay as much as 8.5 gpt. Civ B values 4 horses on 12 horses as 33.3% of its strategics. Civ B wants a minimum of 7 gpt Average is 7.75 gpt, 1.93 gpt per horse. Case 5 horses Civ A values 5 horses as 5 on 9, so 55.5%. 5 horsemen is 5 gpt in maintenance, so 16 spare gold per turn. Civ A will pay as much as 8.9 gpt. Civ B values 5 horses on 12 horses as 41.6 % of its strategics. Civ B wants a minimum of 10 gpt, making the deal impossible. I've only considered the cost of the horsemen maintenance, but maybe their purchase/production could be factored in. Even if civ A is able to purchase 4 horses, it will take a time before it can produce 4 horseman, making this deal less appealing.