Well, I raised the cost of Dark Lord because his ability to passively decrease unrest should allow you to maintain higher tax rates in your provences.
I don't have extensive knowledge about how the economics work, but if he costs 360 and is holy, that would mean he costs 180/15 = 12 gold per turn in upkeep. In the test I did, I was able to raise taxes to 120 % and keep unrest between 2 and 5% over about 12 turns. Don't remember the exact numbers, but many provinces would probably profit from the Dark Lord's presence.
He's also got incredible leadership skills (160/160/160) allowing him to transport an insane number of troops. Basically 480 troops for 360 gold. This 4:3 ratio is the same as ordinary commanders. (40 troops, 30 gold) Realistically, you'll probably never max out his troop capacity and he has only one inventory for items, but he also costs half as much in upkeep, is priest level 1, and decreases unrest.
About the undead summon version, you are probably right. I more or less forgot about him. He is basically just a commander version of the standard immortal undead cavalry. I would give him priest level 1, but im not sure about making standard immortals holy. I'll give him increased leadership skills and look into his cost. Trying to conserve units.
PS, Wauthan -
What was that about heroes? If you have some good ones, I could use them. Generally, i'm fairly lazy. Maybe you can tell by the frequency of my updates, but I do things by whim. I also make stuff mostly in paint because I am too lazy to work out all the components of photoshop. The thing that annoys me most is when it "blends" colors when saving as bitmap. There must be some way to turn that off!