[parcelt]

Another cost to add is that of any commander + chaf army patrolling your province to keep unrest down.

Then again, another saving to consider is upkeep - the upkeep you're not paying for summoned units. In other words, to make a correct comparison against the cost of blood-summoned commanders, a Niefel giant's cost should be expressed as purchase cost of 500 gold (IIRC) + 500/15 * average nr. of turns of upkeep...