SQL Server 2017

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create table COST1(ID varchar(5), Counter int,Cost int);
insert into COST1 values('A', 1, 50  );
insert into COST1 values('A', 2, 30  );
insert into COST1 values('B', 1, 25  );
insert into COST1 values('B', 2, 30  );
insert into COST1 values('C', 2, 30  );
 

create table COST2(ID varchar(5), Counter int,Cost int);   
insert into COST2 values('A', 1, 20  );
insert into COST2 values('A', 2, 40  );
insert into COST2 values('B', 1, 50  );
insert into COST2 values('B', 2, 10  );
insert into COST2 values('B', 3, 20  );
insert into COST2 values('D', 3, 20  );
11 rows affected

Select C1.ID, (C1.cost+C2.cost) as Cost
from
    (select ID, sum(COST) cost from COST1
group by ID) C1

Inner join
    (select ID, sum(COST) cost from COST2
group by ID) C2

on C1.ID=C2.ID


ID
Cost


A
140

B
135


select ID, sum(cost) Cost
from (
    (select id, cost
       from cost1
    ) union all
    (select id, cost
     from cost2
    )
    ) Cost3
group by id;


ID
Cost


A
140

B
135

C
30

D
20