How many units of Botox do I need at 23 years of age for my 11 lines (glabella) in my forehead?

I'm only 23 years old and I don't feel that I need very many units for the area I'm wanting botox in, but all of the research I've found is saying that you need anywhere from 20-30 units for this area which just seems like way too much for me. I don't really want to be spending much more than $100 on such a small area that won't really be noticeable to anyone else but me. Can anyone tell me if its possible to have good results from around 10 units for someone my age?