FormatNumber() converts the number to a string, so the expression concatenates the numbers instead of adding them together.
If you leave out the formatting the variables will remain numeric and the addition will work as you expect.
Code:
$Priv1EDBResult = CDbl(fnGetFileProp($Priv1EDB,Size))/(1024*1024)
$Priv1STMResult = CDbl(fnGetFileProp($Priv1STM,Size))/(1024*1024)
$Priv1 = $Priv1EDBResult + $Priv1STMResult
