Emmanuel Adetutu developed TeeBotMax foldable and portable DIY 3d printer with very detailed build guide. It is open source, the cost to build it is very low and assembly is easy.
Technical specifications:
Build envelope: L 20cm x B 20cm x H 25cm
Printer Assembled: L 50cm x B 40cm x H 53cm
Printer folded: L 53cm x B 40cm x H 18cm
Filament: Compatible with 3mm/1.75mm, ABS/PLA
Print Speed: 120mm/s
Nozzle: J head type 0.4mm
Tested Layer height/resolution: 0.1mm
Power supply: 12v 20a
Build time: one day
Weight: 7kg
Cost to build: 350 - 500 euro range depending on source of parts