This document on the U.S. Department of State website contains frequently asked questions about the DS-160 and mentions the following:
Monthly salary in local currency: Enter your salary before deductions. If you do not receive a fixed salary, you must enter an estimate of your average earnings for a month. If you are a business owner, you should enter an average amount of net income you receive from the business after expenses. If you receive a pension, enter the amount of the pension.
So you should enter your gross income (before tax deductions).
Note: I recommend being careful with some information you find in the document mentioned above. It has been created on 10 March 2010 and has not been modified since. It may not be up to date. It is also located in a folder named "cyprus", meaning that some information it contains may not apply to everyone.