User select the filter criteria current_salary_range is 20000. I will show the results also salary range below 20000
In your Content type Manage Fields, what you need to do is make the Current_Salary_Range FIELD TYPE be a (number) Integer field and not a text field. 


  Note: You will have to delete and create a new field, as you cannot change field type after the field has been created


Then when you add it as a filter in your view, you should get the option of less than or equal to



Note: picture is from Drupal 8, but should be the same in Drupal 7.