You can set the default product quantity within your system to be whatever value you require, normally this would be set to either 0 [zero] or 1.

By setting the Product Quantity on the Product Grids to Zero you will be able to add a line item, and display the sell price of this item, but not have the item included in the totals for the entity.

Note : You are not able to enter a negative value for a Product in the Product Grids.

Note :  You are able to use quantities that are fractions of a whole number (e.g. 0.5. 0.25, 1.5 etc...)

You set the default quantity value by following these steps;

• Settings (top right) > Configuration Panel > Additional Settings
• Look under the Product Settings panel
• Next to the option "Minimum limit for Product quantities on Product Grids" - enter a value
  

When you go into an entity that uses the Product Grids you will notice that the Default Quantity when adding a Product reflects the value you have in the previous steps.