Units tell you more

Botox is packaged as a dry powder.  The physician adds saline fluid to prepare for the treatment.  The amount of saline added determines the concentration of Botox (how many units are in each cc).  There are various reasons why a physician may chose one concentration or another.  However, the real issue is how many units of Botox you received.

Some physicians always use the same concentration so they may discuss cost in terms of cc's. 

You should also remember that Botox is a treatment not just a product.  When considering the cost of treatment you should also factor in the skill and experience of the physician performing the treatment.