Flemish revolution change won’t be enough. Or at least any fixed cost solution will only be balanced for a narrow range of villagers at the exclusion of other ranges. If it is balanced for X +/- y villagers it will not be balanced for villagers > X +y or villagers < X- y. y can only be around 10 if I had to guess.
The issue with the tech is numerous:
- you can mass units cheaper than their upgraded counterpart. No military upgrade has this property at this magnitude (villagers are 40% cheaper than flemish militia). This is the primary reason fixed cost solutions cant work.
- The above combined with losing villagers creates a loan. You loan the player the marginal cost of upgrading the units and they pay it back with lost production. Unfortunately present value (PV) of loans are not easy to calculate when the time horizon is stochastic and can create hyperbolic discounting which makes the PV much higher than you would intuitively think. This amplifies the issue of fixed cost solutions.
- Military units already have an opportunity cost in terms of not making extra TCs and villagers. E.g. you are effectively borrowing from the future by going 1 TC pressure rather than 3 TC boom. This means flemish revolutions present value is higher than even the shorter time horizon would indicate. That is the true cost of flemish revolution is only how far off the optimal villager: military ratio it puts you. If 50 vills: 150 instant military were optimal then the cost is only those 50 villagers being lost.
- villagers are inherently safer while obtaining maximum return. Stone defenses are very cheap for the protection they provide. Military units have a good chance of dying while obtaining maximum return. This makes flemish revolution act on many more units than military upgrades. This means comparing to other upgrades will be ddeceptive.
- the new cost is approximately the same as militia → champion. But due to the above reasons this is too low. Acting like the above dynamics don’t exist and just increasing the fixed cost won’t get you anywhere.
All together these imply that the present value of spotting the player the 10f and 25g for each militia has a much higher present value than intuition would generate. This is why many players can use flemish militia and win the game by a wide margin (many surviving units).
Fixed costs can only work if a fixed number of villagers are converted. There exists an idle military hotkey for selecting them even if the conversion is random. Otherwise you need to have the cost be on a per unit basis or make flemish militia a unit whose value is closer to 50f.
Edit: I want to point out that first crusade and cuman mercenaries are much more balanced precisely because there is a cap on the number of units generated.