How many units of botox or any other botulinum toxin you need is dependent on the areas treated, the strength of your muscles, your facial anatomy and the result you want. The original Botox Cosmetic studies used 20 - 25 units just for the glabella (the frown lines between the eyebrows) and 15 - 20 units for the crow's feet (the lines coming out from the eyes when you smile). Other areas treated like the forehead, chin, jawline, masseters and neck vary also. Patients with strong muscles who want full blockage of the movement may require more to get that effect. On the other hand, experienced injectors often use less to give a natural 'actress dose' so that patients can still make expressions but get subtle brow lift and fewer lines at rest. The price of botox treatment from a given injector may be by the unit or the area. Either is appropriate -- physicians vary in their approaches to billing -- and generally come out similarly.I hope this answer was helpful. Best wishes.