I'm impressed that runs, you're basing logic on a non aggregate field, and then attempting to do an aggregate.
Either of these examples should work, depends on your preference. Note; I have counted your Key value in the first example, I don';t know what it is, so you'll need to replace it:
=COUNT(IIF(Fields!Business.Value = "COM" AND Fields!Priority.Value="High", Fields!YourIDField.Value, NOTHING))
=SUM(IIF(Fields!Business.Value = "COM" AND Fields!Priority.Value="High",1,0))
Thom~
Excuse my typos and sometimes awful grammar. My fingers work faster than my brain does.
Larnu.uk