Mezuzot may be purchased in different sizes. The recommended size is 12 centimeters. I also recommend for those who want to enhance the Mitzva to buy one that is 15 centimeters, for the home's entrance.
More information about the different sizes may be found in the article.
The price of a 12 centimeter scroll is 150 NIS, and a 15 centimeter scroll is 160 NIS.
If you are purchasing a number of mezuzot, you may purchase one 15 centimeter mezuza for the price of a 12 centimeter.