TL;DR: Steam briefly showed options to buy refurbished Steam Decks for the following prices:
- 64gb - $319
- 256gb - $419
- 512gb - $519
The steam page has since been taken down, but will hopefully go live officially soon.
Edit: That was quick, refurbished units available now.



I dunno about DIY at home though.
The video by Valve themselves say not to open Steam Deck, then shows and explains why by showing how tightly coupled the hardware is.
They won’t stop you of course.