Botox® is charged per unit and how much is needed depends on the number and strength of the muscles to be treated. For example, the frown lines between the eyebrows of a 20 year old female may require less than 20 units, whereas that of a heavily browed male may require over 30 units.
 
Botox® is expensive to manufacture and has a short shelf-life which unfortunately increases the price.  However, if used efficiently, it should delay the requirement for more intensive resurfacing or skin tightening procedures later on in life.