It will take a bit more engineering than some optimistic sign-flipping to bring it down to $50 retail (~$10 BOM).
> In my case, the BOM was close to €2500, so be prepared for that.
EDIT: someone pointed out that there are actually some designs available on AliExpress, e.g. https://a.aliexpress.com/_EzMRNIv €250 for one axis (edge) and €500 for a 3-axis (single point).
I think what it needs is simple direct neural network controller quickly trained and tested for cheap motors and high tolerances so it can sort of work if done cheaply.
Recently there was a post here about researchers training NN to fly a chepish very small drone. No messing with PID adjustment.
A similar design (although it balances on the reaction wheel rather than corner after the initial startup jump) has been made for less than 1000 euros and open-sourced:
