Great question! I've seen this exact scenario play out in manufacturing settings, and the ROI calculation is definitely more complex than just comparing equipment prices. From my experience, here's what I've learned about the real ROI factors when moving from relay-based systems to PLCs:

First, the upfront costs are just the tip of the iceberg. Yes, PLC hardware is more expensive initially, but the real savings come from reduced downtime - modern PLC systems can cut downtime by 30% or more through better diagnostics and predictive maintenance capabilities. That's huge for production lines.

Training time is a mixed bag. Your maintenance team will need to learn ladder logic programming, which takes some investment. But once they're trained, troubleshooting becomes much faster - no more tracing wires through complex relay panels. The learning curve pays off in quicker repairs and less reliance on specialized relay technicians.

Maintenance complexity actually decreases dramatically. Relay systems have dozens of mechanical parts that wear out - contacts, coils, springs. PLCs have solid-state components that last longer and fail less often. Plus, you can monitor PLC performance remotely and get alerts before failures occur.

Future scalability is where PLCs really shine. Want to add a new machine function? With relays, you're rewiring panels and adding physical components. With PLCs, you're just modifying the software. This flexibility means you can adapt to production changes without major hardware investments.

The best ROI calculations I've seen include: production efficiency gains (often 20-25% improvement), energy savings from optimized operations, reduced spare parts inventory, and the ability to integrate with modern data systems for better process control.

What specific industry or application are you considering this upgrade for? That would help me give more tailored insights!