Great question! As someone who's been there, let me tell you about the hidden costs that hit you months into implementation when you're already committed:

1. Software Licensing Bombshell: The PLC hardware is just the tip of the iceberg. Siemens TIA Portal can cost $495-$5,000+ depending on features. Allen-Bradley Studio 5000 runs about $1,052 USD/year for the Lite edition. Mitsubishi GX Works3 is $1,000-$5,000 but you might need multiple seats.

2. Training Time Sink: Siemens has a steeper learning curve - expect 2-3 months to get comfortable. Allen-Bradley is more beginner-friendly but still needs serious training. Mitsubishi documentation can be sparse, so you'll spend hours figuring things out.

3. Support & Maintenance Fees: Siemens charges for tech support after warranty. Allen-Bradley's Rockwell Automation support isn't cheap. Mitsubishi's support quality varies by region - you might need third-party help.

4. Ecosystem Lock-in: Once you choose Siemens, you're buying Siemens everything - HMIs, drives, networks. Allen-Bradley's ecosystem is expensive but integrated. Mitsubishi has cheaper add-ons but less seamless integration.

5. Downtime During Learning: Your first few projects will take 2-3x longer than expected. That's lost production time or delayed deliveries that nobody budgets for.

6. Certification Costs: Official Siemens training runs $1,500-$3,000 per course. Allen-Bradley certifications aren't cheap either. These add up fast when training a team.

The real kicker? You often don't realize these costs until you're months in and can't switch platforms without starting over. My advice: Budget 2-3x the hardware cost for the complete ecosystem, and don't underestimate the learning curve time!