Google Sheets - Insert text automatically when cell is not empty

Is there any possible way to insert "000" when I enter values in a cell. There is row called Cost where the value always ends with "000". There are thousands of rows which I need manually enter the values. Lets in Cell B30 if I enter 120 it should look like 120000. Please guide me through or let me know if there is any script that can allow me achieve my requirement.