If I'm buying a PLC, here's few reasons I'd be buying it and not just whacking some isolation/relays on a uC.
- Integration with whatever SCADA system is being used is mandatory now.
- Some form of industrial bus that integrates with everything else in the process. If I had my choice, probably industrial ethernet.
- A known good history in an industrial environment (preferably similar to the environment it's going in, e.g. underground.)
- Expandability
- Software robustness
- Support available from a company that's got good industrial experience, particularly in the sector it's being used in
- Availability of technical persons that know the product line and know how to program it well
- Compliance to all standards as required
- etc
If you take the word PLC out of it for a moment, and don't try to compare it to a real industrial automation PLC, then it's nice project at a reasonable price.