Well, you can buy replacement packs for Dyson on Amazon; for example, a replacement for SV04 pack is $39.99, and currently (heh-heh) the capacity is listed as 5000 mAh. I see 4000 mAh replacement packs on eBay for only $28.
Compared to some battery packs for tools, this price is quite reasonable. To rebuild yourself, you’ll need six cells to get 21.6 volts, and I see six cells at 4000 mAh each on eBay but for $37, so there isn’t much incentive to rebuild them yourself unless you’ve got a great deal on cells. And they’re probably flat-tops that need to be welded.