In the end this depends on your definition of "fair". What percentage of your generated production do you think is fair for the company to take? 95%? 50%? 10%?
That depends on the value of your generated production, among many other things, and ultimately isn't the right question to ask.
Can an employee obtain better employment terms elsewhere (which is a complex concept to define in itself)? If so, they are underpaid, if not, they aren't.
You were talking about exploitation. Using the fact that the employee cannot obtain a better employment elsewhere to extract as much of the production or value from the employee smells a lot like exploitation to me.
If an employer offers an employee $100 per hour, and the next best offer that employee can obtain elsewhere is $90 for an otherwise equivalent job, should the employee take that job for granted? Is the employer exploiting them with their pay rate?
That would be the case in an idealized world. As with everything this depends on the circumstances and the economic activity of where the person is living in. I guess that with the north american eyes it is the employee's fault if the employee cannot find some other job since the only constraint for doing it is the personal drive. But there are other economical/educational constraints that don't allow people to have the necessary mobility for your example to be efficient and accurate.
Put down the Ayn Rand BS books. What if the employers make 10k per unit of work while they pay you only $10 per unit of work and they have all talked to each other to never pay more than $10? What do you do then? Complain? Go to court? Who do you think has more influence over the politicians/courts? You making $10 or your bosses that are all millionaires because of your severly underpaid work?
