Working with tidy/long data Grand Total Incorrect When working with a GroupBy in a Variable.

You can change the code to this:

Total Costs =
IF (
    HASONEVALUE ( 'Attribute Slicer'[Attribute] );
    SUM ( 'Attribute Slicer'[Detail_NET_AMOUNT_AMOUNT] );
    CALCULATE (
        SUM ( 'Attribute Slicer'[Detail_NET_AMOUNT_AMOUNT] );
        FILTER (
            'Attribute Slicer';
            'Attribute Slicer'[Attribute] = SELECTEDVALUE ( 'Attribute Slicer'[Attribute] )
        )
    )
)

oops, the [Total cost]-measure was a bit more complicated than necessary. It should be written like this:

Total Costs = 
    CALCULATE (
        SUM ( 'Attribute Slicer'[Detail_NET_AMOUNT_AMOUNT] );
        FILTER (
            'Attribute Slicer';
            'Attribute Slicer'[Attribute] = SELECTEDVALUE('Attribute Slicer'[Attribute])
        )
    )

If you want the average across an attribute you could use this code:

Average Total Costs = 
AVERAGEX (
    CALCULATETABLE (
        DISTINCT ( 'Attribute Slicer'[value] );
        ALLEXCEPT ( 'Attribute Slicer'; 'Attribute Slicer'[Attribute] )
    );
    CALCULATE(
        SUM ( 'Attribute Slicer'[Detail_NET_AMOUNT_AMOUNT] )    
    )
)

But this will depend a bit on how you want to use it. Also think about how to handle blank values; should blank values count as 0 in the average, or should the average just be over the attribute values which have a value

It all depends on what you want to do. As you can see in your screenshot, [Average rate] is changing with each 'Attribute'[Value]. This is because your code now calculates the average rate for each 'Attribute'[Value]. E.g. look at the second line of the table Draft/revise. On this line the filter context is 'Attribute'[Value]="Draft/revise". This means that in this line, with your average calculation, you are calculating the average of rows from your table where 'Attribute'[Value]="Draft/revise". If this is you desired output, your code is perfect.

If you want to compare rates from, say company, to the average of all companies, you will have to use DAX-functions to alter the filter context, like the example code I sent you.

Cheers,
Sturla