So if each employee brings in an average of $500,000 a year and take home $25,000 a year then that is roughly 5% of their total sales. Ya thats a little low. However, they could just increase the percentage to like 7% which would bring in roughly $35,000 a year for the employees. That would be a dramatic jump in pay. That is if they were being paid commission.
The assumption you are making is that the product is selling because that particular retail employee did something another could not. Apple sells products primarily because:
1) Engineers designed them well (software and hardware)
2) Marketing positioned them well in the competitive landscape
3) Retail policies and training trained the retail employees to follow Apple's retail methodology
4) Support policies and customer service policies have made customers feel secure buying from Apple
Certainly there are great retail sales people, but if the retail staff was highly skilled and irreplaceable and the primary reason for those sales then they would deserve a bigger chunk of the pie. So long as the primary marketable skills of the retail employees comes from Apple's internal training program then Apple is going to know they can replace an employee with another one if given the right training. This is called basic supply and demand. If good retail employees were in short supply then their value would increase.
It seems to me that skilled jobs like teachers should be getting paid far more than retail staff and they only start at $25,000 to $35,000 per year in many states (see http://www.teacherportal.com/teacher-salaries-by-state).
As long as Apple is paying more than the average retail shop for the same kind of work then they are being more than fair.