The effect of Botox injections tends to last anywhere between 4 to 6 months. The number of units of botox used for injections depends on the site of treatment. It can vary from as minimal as 1 unit each side for the injection of hypertrophic pretarsal muscles ( that's the muscle bulge below the lower eyelid when you smile, and these are not eyebags which need a different sort of treatment!) to as much as 80 to 100 units in total for the injection of hypertrophic masseters (that's the muscle on each side of the jaw which in certain cases is overdeveloped, forming a square jaw that is a bit too masculine in women for most people's liking).
The cosmetic cost (that is, the cost of Botox treatment) is usually calculated at between $20 to $35 per unit, depending on the site, the degree of skill needed and the location of the practice. Suffice it to say that in the USA prices may be a bit high compared to certain parts of Asia, such as Thailand, where prices are very reasonable.