> Like solid machines sub-$100.

You can get 3D printers from BestBuy(!) for $200 retail. At that point, the cost of the filament is going to quickly exceed the cost of the machine.

At the $200 price point, your Bill of Materials is roughly $65 (about 1/3 of the retail cost). I challenge you to buy the raw materials of a 3D printer for under $100 let alone $65.