Aldric Negrier developed this DLP projector based open sourced 3d printer. It looks easy to assemble and cheap to make. It all depends on how cheap can you find a suitable projector.
Roobee One is an SLA DLP 3D printer inspired on the Cristelia - SLA/LCD 3d printer and the Vulcanus MAX 3D printer.
It is built out of 20x20 mm aluminum profile chassis, It has an adustable print area of 80x60x200 mm up to 150x105x200mm build volume using a ACER DLP projector.
The open-source machine is called RooBee One because it is red color just like a Ruby gem.
RooBee One presentation video:
Detailed build guide can be found at:
Cristella 3d printer (open source, daylight):