I refuse to believe this is the only way to make it cheaper. Standardization and modular components could help. Might not be cheap enough. But surely we can be more imaginative!
My intuition is that a small radio from the 80s is sufficient. Has relatively small size and passive speaker. Small passive speaker and small enclosure are cheap aka not 100+ USD. A tonie box sounds fine. Can I diy reproduce one similar at low cost (less than buying 50 USD)?
I've spent countless hours searching for a small, good quality passive speaker. In a roughly 4 inch cube size. That comes with an good (acoustically) box/enclosure. Plenty exist in various forms (kits) but I never know if they sound as good as even laptop or phone speakers. Which is a low bar, but still can't find good reviews.