Hi
Do not place your text field directly into the report, in field explorer you will find formula fields and right click on it and create a new one.
Now here enter a formual to associate a condition to it.
Example :
If {databasefield_Country} = "USA" Then
{USA_SALES_DATABASEFIELD}
Else
0
'@' symbol represents that it is a formula field. there is no significance for this.