It's a proprietary form factor, so you're gambling on replacements being available down the line. I don't think it'll be easy to rebuild the battery pack without compromising the casing.

Given the battery specs and the form factor of the products, they could have used a 14500 cell that retails for $5. That's not as much recurring revenue as charging $80 for something proprietary though.

3d printer and couple of cells and I guess a board will get the job done.