I bought one of them fast framer kits from northern tool. Where you use straight cuty 2x4 and made mine 8'x8' with 6 foot walls and covered it with greenhouse film from farmtek.
NH 1630 W-7308 FEL/ Kubota L4630GSTC W-LA853 FEL WQ/A-CC 2544
My greenhouse is over three years old. Pvc frame with plastic over the top. Took about two weeks to build from scratch. Has heater, intake fan and exhaust louvers. Twenty two by thirty two. Four rows of shelves in the inside. Have started over fifteen thousand plants one year. The biggest expense in any greenhouse will be the fan, louvers and heater. The shelving frames with wire can add up fast.