I can't speak to your specific situation, but the muscles in men's faces are larger than women's, and they frequently require more Botox. So it's common for the cost of Botox in a man's forehead to be more than a woman's forehead. My guess is that the increased cost just reflected his larger muscles there.

Link to Guide to Botox

Link to Dr. Irwin's video demonstration of Botox and Juvederm