The Prius uses ~250Wh per mile from what I've heard. So, it looks like they're adding an additional ~7-8kWh. Thats really not much at all. The ForkenSwift has a 10kWh pack. It should only cost a few thousand more for the pack itself. I estimate $3k in Lithium at prices available to us, they'll obviously get it cheaper. The rest of everything is already there, it just requires some programming changes.
Edit: Wow, my math was way off haha.
14 miles @ 250 kWh mile would be 3.5 kWh. It seems, according to Ben's post, that people are averaging 370 Wh/mile with the Prius, not very good compared to our other low speed DC conversions around here!