In a company of "white-collar" professionals, what is the average cost
of managing an employee, as a percentage of his/her respective salary?
 Particularly interested in small- and medium-sized businesses, up to
250 employees.

For fee earners I've always reckoned that you need to bill them out at
three times their salary.

 X for salary
 X for social security, office space, kit, expenses, insurance, admin
 X for bothering to employ them in the first place

It depends on the industry, and what they are doing, but my 'feeling'
for the subject is that it is hard to directly manage more than 10
people without setting up an intermediate structure.

It depends on the industry
- a contracting organization can run on 2x - or less if it is high
volume, since there is little feedback ie: it is top to bottom
communication