How to use JOIN and SUM function to Calculate the total of query returned column?
You can use grouping set also 

declare @JobRequiredProducts table (jobId int, productId int);
declare @Product table(productId int, productUnitPrice decimal(8,2))

insert into @JobRequiredProducts(jobId,productId)
values(1,4),(1,5),(1,6);

insert into @Product(productId, productUnitPrice)
values(4,175.99),(5 ,100.00),( 6 , 125.00)


SELECT  j.jobId,
        j.productId,
        SUM(p.productUnitPrice) AS 'Total'
FROM    @JobRequiredProducts J
JOIN    @Product p
  ON    p.productId = j.productId
WHERE   j.jobId = 1 
GROUP BY  ROLLUP ((j.jobId, j.productId));


with grouping sets

SELECT  j.jobId,
        j.productId,
        SUM(p.productUnitPrice) AS 'Total'
FROM    @JobRequiredProducts J
JOIN    @Product p
  ON    p.productId = j.productId
WHERE   j.jobId = 1 
GROUP BY  GROUPING SETS( (j.jobId, j.productId),());


output:

jobId   productId   Total
1       4           175.99
1       5           100.00
1       6           125.00
NULL    NULL        400.99


dbfiddle here