Designing signal conditioner to convert current source to voltage output
Are the high costs due to protection against spikes in the input/other extreme events?

Yes, this would be a standard expectation in industrial control systems. Many also include isolation between the input and output signals and this will push the cost up significantly. This eliminates ground loops and allows, for example, the PLC to be electrically isolated from the sensor loop.



simulate this circuit &ndash; Schematic created using CircuitLab
Figure 1. Conversion doesn't have to be expensive.
If you can tolerate 2 - 10 V from the conversion then a 500 Ω precision resistor will do the job. These are available in 0.1% tolerance from the big suppliers such as RS.
Just as the 4 mA gives a &quot;live zero&quot; (which means that you can tell the difference between a sensor giving a zero indication as opposed to a sensor that has become disconnected) the 2 - 10 V can be used in the same way if you can set the scale and span of the input you are feeding.