As a general rule, T1 military and diplomatic units provide more defense per g/h tha their T2 counterparts, but the T2 units offer more Attack bang for the buck. This isn't always the case though.
Also, you refer to "cost per defense." Are you sure you don't mean defense per cost? Measuring the amount of attack/defense a unit provides, divided by their upkeep cost in gold, is a more effective way to look at efficiency.
Looking at Human spear units, a Militiaman(T1) has 7/11/4/11/13 with a 1 g/h upkeep. A Pikeman(T2) has 16/17/11/17/29 and a 2g/h upkeep
(The stats go ATK/Spear DEF/Bow DEF/Sword DEF/Cavalry DEF). Now we look at the upkeep costs. It costs the same g/h to have 2 Militiamen as it does to own 1 Pikeman. So, if we compare 2 Militiamen to 2 Pikemen, we get:
Militiaman: 14 / 22 / 8 / 22 / 26
Pikeman: 16 / 17 / 11 / 17 / 29
Note this shows the value of 2 g/h worth of units. To get the actual stats per unit of upkeep cost, divide all stats by 2. Probably obvious but worth clarifying.
Thus, based on cost efficiency, you will gain more Spear and Sword defense by training Militiamen, but more Attack, Bow and Cavalry Defense by training Pikemen. This is not groudbreaking by any means, there's probably a tutorial somewhere that shows this. Hope it helps you out though.
EDIT: Oh, I see you are referring to the initial resource production cost, not upkeep cost. My bad.
Edited by Hadus - 12 Jul 2012 at 03:16