and ... uh... steroids?
Stimpacks anyone?
The conversion rate seems like a very good, and simple, idea. it does put everything in perspective. A basic stat configuration to help give a comparison would go far, if not a long way, in helping us see what we're getting ourselves into. Both a Heavy Infantry and a Cavalry models, if i'm reading you right, might have the same attack and defence stats but without those horses the cavalry unit would be comparibly weaker and be just as slow as the Heavy Infantry unit. Thus making the only difference between the two units is that the Cavalry unit is much faster and more manueverable. Of course, depending on the quality of the horses or whether there is an option for factions, previous or player created, to have specific animal husbandry bonus(es), like boosted attack, defence, speed or price reduction, a cavalry unit may be more cost (both gold and training time (turns)) effective than said infantrymen.
Another aspect to take into consideration is weapons. Lets look at the variables between a spear, an axe and a broadsword and ignore the other weapon varients (like halberds, pikes, bows, two-hander weapons and pitchforks) for the time being.
Cost and Quantity of Materials Needed: the amount of metal required to make one basic broadsword would be the equivalent, oh lets just say, 5 basic spearheads and axes would fall somewhere in the middle. From this alone, spearmen would naturally be cheaper to produce and maintain than swordsmen.
Training: Spears, being straight forward, don't require much training in their use; remember stick'em with the pointy end. Axemen on other hand need more training time so they don't accidently turn themselves into eunichs. And swordsmen, well, require even more extensive training because of the weapon's versatility.
Attack and Defense: I consider spears to be more of a defensive weapon (in that they keep an enemy at a distance but limit attack options as they are best used as thrusting weapons). So as a balancing factor we could make spears have a weaker attack but an okay defensive ratio. Axes are all about aggression and hitting the enemy hard but give little in the way of defense. And swords being naturally more versatile would have a more balanced ratio.
Or put in numerical terms of AP (Attack Points) DP (Defense Points) TN (Turns Needed) UM (Units of said Metal) and $ (cost) to train a single Spearman, Axeman and Swordsman. (Just the requirements for the weapons alone)
Spearman: 1AP 3DP 3TN 1UM 5$ Axeman: 3AP 1DP 5TN 2.5UM 7$ Swordsman: 3AP 3DP 7TN 5UM 10$
Of course, this is just a rough sketch that could help make players decide on how they want their armies configured just from weapon selections alone. And you probably noticed, I didn't add weapon characteristics on purpose like Spears bonus against Cavalry (which I consider just a tactical advantage not a Scissor's beat Paper absolute, cause under the right circumstances a force of cavalry could wipe out a force of spearmen).
Oh and I did checkout Frogboy's latest info tease...after you mentioned it.
I guess it would have been funnier if i had posted this a day prior. but at least he answered two of this post's questions.