How do you measure "X", anyway? During the whiteboard interview process? We all regularly discuss how subjective and broken that process is. Do you measure it after the hire, by tracking the number of story and task tickets closed? Now you just have people gaming the system, with a sprint board full of bullshit.
Everyone wants more money. No one wants to work for less than a certain baseline. Everyone's pissed when they find out what their peers make. So if you're an employee who thinks you're underpaid, then test the market. And you're an employer who routinely offers great candidates many multiples of the market rate, then please let me know where I can send my resume.