If currency(summary field) is blank formula help

I have a currency summary field in my invoice table that shows "our cost" which is summarized from another table in out APP.  

I would like to create a formula checkbox field that will be checked if the "our cost" field is blank in my INVOCIES table. Is this possible to do?

I have tried several formulas option in a checkbox formula field, the NZ function, as well as the Null function using another field I created and converted our cost to a text field. 

I need to use this missing our cost field for several other things as well, but cannot get the formula right.....

Thanks