Thanks, adding the TOTAL qualifier inside the MAX fucntion worked.
I was a little confused about about when exactly the TOTAL qualifier disregards dimensions.
It appears that the TOTAL qualifier disregards dimensions unless aggregation is explicitly used in the expression (as was done with the AGGR function).
I'm curious, though, why this wouldn't also work: MAX(TOTAL SUM( TOTAL <Group> Cost)). Doesn't the <Group> I've added in the SUM also do aggregation?