Custom units/commanders - assigning cost ?

[velk]

I have been playing around with some custom units, but I have been having great difficulty coming up with a good way to assign resource and gold cost to them. How do you guys decide on these values ?

As an example, take the niefel jarl. The most expensive unit in the game, with excellent combat ability combined with quite powerful magic and a potent special ability (cold aura).

What proportions of the 500 gold would these values be weighted at ? How much should they cost if I, for example, removed their magic skill entirely. How about the cold aura ? Adding regen to it - worth more if it's combined with the cold aura or same amount regardless ?

Or for an even simpler one - let's say I double the precision on man's longbow unit. Would it be worth more or less than double it's previous cost ? On the one hand, against a big swathe of units, it would most likely do exactly the same damage as it's less accurate predecessor, on the other hand against say a single attacking mage it would be orders of magnitude more powerful.