Maaser, deducting ten percent from ones income, and giving it to Tzedoko, charity, or to other Mitzvos, is a very honored custom which has been accepted amongst Klal Yisroel. For a business owner, Maaser is only calculated after all work-related expenses have been deducted. For example, advertisement expenses, the price of the electricity in the business place, employees salaries, etc., should first be deducted, and only after one has the net amount are the ten percent taken as Maaser. However, expenses which are home-based, such as those needed to run the business owners home, not business-related, cannot be deducted. In a case where a person is tight, there may be room to consider first deducting those expenses as well. For a personalized ruling, please consult a Rov.