I have a Shark pack and I know, more or less, what it takes to build a pack. This wouldn't be a huge amount of labor, for someone who knew what they are doing. The packs, empty, are not that expensive, from China. The price of the cells is coming down. You can buy nice Panasonic GA cells for $6 each.
If you add up what these parts and the labor cost, it is closer to $400 than $800.