The term "fair" is subjective and leaves much room for interpretation. Before we can determine what would be "fair", we must quantify the parameters of the concept. Someone's definition of fair would be determined be their work ethic. For example, some people expect that they will be working the standard 40 hour workweek, and overtime as needed, and that they will be compensated with wage and bonuses as earned when they go above and beyond their job description. Someone without a strong work ethic will expect more pay for less work. So, it is safe to define "fair" as halfway between the two personalities.
It would be fiscally irresponsible for the employer to pay a low-level, part-time employee a wage that would support an entire family on one income. That just isn't good business practice. The higher levels of the company would expect a commensurate increase in wage over their underlings, and this would bankrupt the company. The trickle-down effect would cause the company to raise the price on their product or service. This can easily collapse the company, as it is no longer able to offer competitive rates to the market, and the market will naturally eliminate the company. No one wins. So, in short, fair could be quantified like a negotiation: no one gets everything they want. Both the employee and employer sacrifice something to get something.
Now, to determine fair compensation, research must be conducted. Research is key. Find out the median income and the average family size, and incorporate this into wage consideration. Another consideration in the average education. Ascertain the tax rates and determine how much of the wage is actually taken home by the employee. What is the average cost of living? A police officer in New York City starts out making a whole lot more than a police officer in Montgomery, Alabama. Is this fair? Yes, because the cost of living is significantly higher in NYC. There are variables to identify, and it is only through due diligence that they can be discovered.