MySQL 8.0

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CREATE TABLE T(
  cost	int,
  columnA varchar(50),	
  columnB varchar(50)
); 


INSERT INTO T VALUES (250,'Foo','Bar');
INSERT INTO T VALUES (200,'Foo','Bar');
INSERT INTO T VALUES (150,'Bar','Bar');
INSERT INTO T VALUES (250,'Foo','Bar');
SELECT 
    SUM(cost),
	(
        SELECT tt.columnA
        FROM T tt
        GROUP BY tt.columnA
        ORDER BY COUNT(*) DESC
        LIMIT 1
    )  AS columnA_dominant,
	(
        SELECT tt.columnB
        FROM T tt
        GROUP BY tt.columnB
        ORDER BY COUNT(*) DESC
        LIMIT 1
    )  AS columnB_dominant,
    (
        SELECT COUNT(*)
        FROM T tt
        GROUP BY tt.columnA
        ORDER BY COUNT(*) DESC
        LIMIT 1
    )  / COUNT(*) AS columnA_percentage,
	(
        SELECT COUNT(*)
        FROM T tt
        GROUP BY tt.columnB
        ORDER BY COUNT(*) DESC
        LIMIT 1
    )  / COUNT(*) AS columnB_percentage
FROM T t1


SUM(cost)
columnA_dominant
columnB_dominant
columnA_percentage
columnB_percentage


850
Foo
Bar
0.7500
1.0000


SELECT SUM(cost) OVER(),
       FIRST_VALUE(columnA) OVER (ORDER BY counter1 DESC) columnA_dominant,
       FIRST_VALUE(columnB) OVER (ORDER BY counter2 DESC) columnB_dominant,
       FIRST_VALUE(counter1) OVER (ORDER BY counter1 DESC)  / COUNT(*) OVER() columnA_percentage,
       FIRST_VALUE(counter2) OVER (ORDER BY counter2 DESC) / COUNT(*) OVER() columnB_percentage
FROM (
  SELECT *,
         COUNT(*) OVER (PARTITION BY columnA) counter1,
         COUNT(*) OVER (PARTITION BY columnB) counter2  
  FROM T
) t1
LIMIT 1


SUM(cost) OVER()
columnA_dominant
columnB_dominant
columnA_percentage
columnB_percentage


850
Foo
Bar
0.7500
1.0000