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?
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.