Power used by a PLC for communication
It seems the manufacturer is telling you outright what the worst case power dissipation can be, which is 12 W.  Why is that so unbelievable?

Look at the datasheet, and 12 W is probably the worst case when everything is on or set to cause maximum dissipation in the PLC.  Perhaps they also give the worst case minimum power dissipation when nothing is connected, maybe called "idle" or something like that.  You can probably make some reasonable guesses where between these your application is.

As for the power dissipated by RS-485, that is probably not relevant.  The manufacturer is already telling you the maximum the PLC itself will dissipate.  If the PLC is one end of the bus, then the maximum PLC power dissipation already takes that into account.  Figure the other end is 5 V across 120 &Omega;, or about 210 mW.  However, that won't count towards your heat budget if it is not in the same box.

As for why the PLC has 0% efficiency, the whole concept doesn't make much sense.  Efficiency of what compared to what?  It's not meant as a power conversion device.  Eventually, all the power supply power into the PLC is going to end up as heat somewhere.  Some of it may be outside the box when the PLC is driving remote relays and the like, but most of it is going to be inside the PLC unit.  At least that's the worst case the manufacturer is telling you to assume.