Well the game has a set # of resource cost for each item in it. However the Developers lost the napkin they had it on so we don't know the exact numbers.
You'll also note that the effects of statistics are not linear. So if, say, you double the precision of a longbowman, he is not twice as effective, but probably 4 or 5x as effective. Thus there is no simple way to measure that effect. The best way is to look at the base game units and take a compariable look at gold cost vs unit specific stats/abilities and come up with a balanced answer for it.
P.S. The reason Niefel Jarl are 500 gold is they are insta-supercombatants. They have Water Paths so can go underwater and have access to Quickness and if given a small set of equipment, become terrors in and of themselves.
[ January 22, 2004, 03:50: Message edited by: Zen ]