Actual PLCs in the wild are industrial cabinets with a lot more capacity. Price-wise if you have a DIY project in mind you're better off buying a RISC-V microcontroller, under 10 bucks including board, with vastly more IO and processing power. Still not enough to replace one of those industrial cabinets, though, especially when it comes to IO capacity, do you have any idea how many sensors and actuators rollercoasters have.

This thing is so limited you have to engineer your problem to fit. Maybe something like switching your doorbell from normal to quiet to silent to flash lights. On the upside it's dead-simple.