Am I Overpaying for Botox?

There is probably no more diificult subject to get a handle on than Botox fees. In general, you need to know what you are paying per unit in order to make comparisons. And, as Dr. Lupo pointed out, you would not necessarily get the same amount each time. Comparing the skill and experience of your injector is more difficult, and it is not a good idea in my opinion to shop for the lowest per unit price. Some practices have loyalty incentives, so to know whether you are really getting a 10% discount you need to know more specifically what the fee structure is. It does sound like you are getting good care.

Dear Choyce
You will definitely pay more for a more experienced physician or surgeon injector. In many cases, the extra skill and knowledge makes a huge difference in the quality of the service.
I don't agree that with repeat treatments, you require less BOTOX. As we have gotten better at getting more aesthetically pleasing results with BOTOX and not simply treating to the point of paralysis, these results don't seem to last longer and longer. Much of this is how you feel about your treatment. It is my experience that more dramatic treatment results usually prompt patients to come back sooner for retreatment because they are enthusiastic to maintain a great look.
On the other hand, you are in Stockton, California, which is a very depressed market. It may very well be that with a little effort, you can find another highly skilled physician, be just as happy with the results (or more happy) and spend much less on the service. You are describing a pretty generic type of treatment. I have a highly specialized, patent pending microdroplet BOTOX technique that produces a forehead lift. Patients love the treatment and I am in West Los Angeles and I charge less than your doctor does. I am not suggesting that you need to come see me but I support your feelings that perhaps you are paying too much.
Do some research and try another physician in your area. Be aware that you may discover that after trying other physicians, that it is more than just the BOTOX service you are paying for. The staff and doctor who is treating you may be more comforting than other offices, spend more time than other offices, and in general provide more care. You may decide that even though you can get the same basic service for less elsewhere, these it is this higher level of treatment that you appreciate and sometimes this is worth paying for and sometimes it is not.

Every one charges differently for Botox.  Typically it is between 8 and 14 dollars  per unit. So you do the math to see if you are paying too much.