Is There a Way Around the Required 50 Units of Botox?

Technically, Botox is labeled as a "single use" vial. This has to do with the way it was approved for use by the FDA during the clinical trials. Most physicians use it "off label" by using one bottle for more than one patient. If done with new syringes each time and a sterile technique, no deleterious effects have ever been demonstrated or published. By using the vial for a second patient, no harm is being done and you are trusting that your doctor is using proper patient safety precautions.
 

If you have always had the exact same area of your forehead treated with 20 units of Botox and obtained a good result, I would stick with that dose.  If you are adding other areas, such as the frown lines or crow's feet, you will most likely need more than 20 units.  If your injector is requiring you to pay for 50 units and only using 20, I would seek out a different physician to treat you.