For a long time, public signs have been the focus of many researchers and scholars. They probe into this kind of meaningful and special language from different perspectives by adopting theories from a large scale of areas. However, the researches were mostly limited in several hot fields, like linguistics, translation, culture or travelling. Nevertheless, in the year of 1965, Jacob Marschak(1965), an economic professor in the University of Los Angeles presented a theory, which ted the researchers eyes to a brand new region, which is language economics. In the following paper, the author will combine the theory of cost and benefit with public signs. Two types of public signs will be particularly analysed using the theory of cost and benefit that belongs to language economics.