Actually, my previous graph was a bit of an exageration. To be honest, medium units are good too, and are far better in the case of a player-fought battle.
This graph is more accurate at describing cost by strenght. The green curve is the ratio of power/cost, and I've set blue and red line to same scale.
Low levels units are really the best (especially lvl 0... sadly it doesn't exist
) but the difference between lvl 10 and lvl 30 is a bit under 10% of power/cost.
This makes the game far more balanced that what I initially thought, since quality army is easier to move... that's a true dilemma
The simplest rule to understand is that quality woulld equal cost if the wage paid by a unit was thewage actually paid by a unit one level under. (What I mean is that if a unit of level N received the wage of a N-1 unit, quality and cost would be directly proportionnal : we always pay "one level too much"