This doesn't address the issue specified above. In this example, you already provide unique value to your employer at $20/hr over what they're paying you.
If you have no recourse when they value your work at $40/hr, what recourse would you have when they value your work at $60/hr but still pay you $20/hr?
Being a more valuable employee works great in combination with exercising leverage over your employer (quit-threat, striking, competitive labor market), since those threaten to take that value away from your employer if they don't play ball.
If you have no recourse when they value your work at $40/hr, what recourse would you have when they value your work at $60/hr but still pay you $20/hr?
Being a more valuable employee works great in combination with exercising leverage over your employer (quit-threat, striking, competitive labor market), since those threaten to take that value away from your employer if they don't play ball.