D:OS 2 will have much more text than D:OS. Until the scope of how many origin stories, etc, are being added is known, it will be difficult to determine how much voice acting will be feasible.
Based on the development of D:OS and experience with voice acting in Dragon Commander and Divinity II, I'm pretty sure they can make a safe and average guess with a margin.
So they should have a value of how much it costs per thousand voice acted words or so.
