I hoping to Stat up Dark Eldar as PCs for my black crusade game. I'm not sure what value to cost the Unnatural Agility(3) at.

You might not need to value UN-Agi at all. Given the stats of Kabalite warriors in the book... I would just say they start with everything a kabalite warrior has, and 500 xp.

You might need to stat up some DE weapons though.

Well, i was hoping to create Dark Eldar Archetypes from stratch.

Ok. Same thing goes really. Use the kabalite warrior as a base line for the race, in the same way that CSM get baseline talents and traints. Maybe reduce some of the skills down to trained from +10. Then create new archetypes with skills and talents you want, plus starting gear and a unique special ability for each. Players get 500 or 750 xp at creation to use. Dark Eldar always count as aligned to Slaanesh, and as a possible rule they always reduce their corruption gain by an amount equal to double their infamy. Dark Eldar do not gain mutations like normal characters, but they may mutate if affected by powers like Bolt of Change and anything else that forces a mutation. Their corruption reduction doesn't apply if they haven't consumed a soul for X amount of time. Dark Eldar weapons are considered exotic to anyone not a dark eldar or eldar. Additionally, some dark eldar weapons may harm a wielder if they aren't trained in its use, in the same way an Executioner Sword does [inquisitors handbook, some page I can't remember].

How does that sound?

Give it to them for free. The starting Legionaire archetype gets 8 points worth of Unnatural Attributes, a hell of a lot more than the 3 the Dark Eldar would get.