Can Someone Tell Me How the Charges Are Made for Botox?

There are different techniques to injecting botox or dysport to get different results. Also the size of the area or how dynamic the muscles are helps determine the amount of botox to use. Men have larger foreheads and need a lot more botox than women as a rule. What is important is the results you get so don't go cheap or you are wasting your money and will pay more in the long run to get touch ups which are not free.
When your doc charges by the unit, you have no idea going into it how many units your doc feels you will need to accomplish the results you want. You yourself will under estimate the amount of units you need by such an amount that you will be shocked when you pay your bill, whereas if you know you want the crow's feet or frown lines or forehead creases done, then if your doc charges by the area then you know exactly what you are going to have to pay and you can plan accordingly with your money whether you can afford it or not right now.
It all boils down to your results and your doctor patient relationship. Even if he does a great job most of the time, there will be a 1 out of 10 times when the botox doesn't seem to work as well and a touch up is needed even in the best of your hands.

Generally, clinics provide a per unit price, and you simply multiply that amount with the number of units administered during treatment. I haven't heard of charging per syringe.

Dear Jlosc
BOTOX (and now Dysport) are very technique dependent services. The standard treatments are only so good. Advance injectors will individualize treatments. At the low end of the market, injectors are advertising BOTOX by the unit at prices that are less than the cost of the material. Obviously no one can stay in business giving things away so there is usually a check (including giving you less BOTOX than you pay for). Other office will charge by the area. Recognize that there is no general agreed definition of what is an area. Certainly you want to know exactly how much BOTOX you will be receiving whether the office charges by the area or by the "syringe". Treatment priced by the injection is just another variation on this theme.
It is my personal belief that not all BOTOX treatments are the same. As an injector, I do not expect that my patients will know how many units of BOTOX they will need, or what areas they should be treated in. It is my approach to offer BOTOX as a service. We also tell the patient exactly how many units of BOTOX they are being treated with so that if they get service elsewhere they know what was done before. It is worth noting that you are best served if you are treated by the doctor and look for a doctor from the aesthetic speciality core: Dermatology, General Plastic Surgery, Otolaryngology, Ophthalmology.

It varies significantly from office to office. The most important aspect is to choose a reputable and experienced injector. Some will charge you by area and some like us will charge you by unit.  In our case, it is $12/unit. 

I agree that you should be charged by the UNITS, not the area. Some areas simply require more units, and that is directly proportional to our cost. On the other hand, there is usually remarkable consistency in the number of units needed--usually around 30 per area. We do quote by the area for that reason. It is very consistent for our staff and patients...no ambiguity. As a patient I would always wonder: did I "need" more units because I was dressed well and looked like I could afford it? Of course, if someone truly needed LESS units, I would adjust my fees lower. I NEVER adjust them higher!