[cheald]

To elaborate a bit on this, the term economists like to use for this is "alternative choices". The minimum price of the assistant is somewhere around how much it would cost to hire and train someone to replace her - that is, her minimum price is set by the price of the alternative choices that the employer has. After all, it is irrational to hire an assistant at $2X who will produce Y output when you could hire an assistant at $X who will produce Y output.

On the other end of the scale, the maximum price is value of her productive output to the company - it is irrational to pay her more than she can generate.

Negotiation is the skill of convincing your employer that you're harder (ie, more expensive) to replace, thus propelling your pay closer to that maximum.