Sounds like a job for an analog multiplexer. I'm not familiar with the design of such circuits but they're pretty common I think. For example, they're used in cheap multi-input video frame grabbers. 4 inputs -> 1 A/D via an analog multiplexer.
Of course you would need to figure out a way to synch it with the PLC. Meaning you would need to control which input of the multiplexer is active using an output from the PLC so that you know which input you are sampling. The timing of that could be tricky, especially if you can't tolerate a slow sample rate.
Another option (other than what support mentioned above) is to get a serial analog input module. ICP has quite a selection of these but I don't know what your definition of cheap is.