I understand the pricing policy. If you are selling to a large industry customer you have to provide support and have product liability. They have to be at such a price that they can just swap out any problematic unit. Also the employer has to have employee liability and that would probably preclude buying from an 'unreliable ' source.

I wouldn't mind if anyone could tell me the modular functionality of such a unit, and what qualifies it to have the title 'safety relay'.

I'll probably include one in my next build.