you're not paying $50 for cardboard, you're paying $50 for R&D to design a cardboard pinball that can easily fold up (and swap playfields), and laser cutting time. I don't see any reason anyone couldn't make their own pinball out of cardboard. I'd sooner try to pickup a used zizzle for $50-$100 before I spent it on cardboard.

I think it would be interesting for someone to make an open source fully mechanical pinball design with return compression springs (instead of rubber bands), maybe you could layer cardboard and use elmer's glue to build up the right thickness and even use real pinball components (3d print components from pinballmakers.com in cases you don't have spare parts to save money).

Nice job!