Apportionment Query/Best Practice

[Comfy]

Hi,

I'm slowly but surely learning SQL etc and was hoping someone could advise on the following:

I have these two tables:

Excel 2010
	A	B	C
1	ID	Site	Cost
2	1	A	£100.00
3	2	B	£200.00
4	3	C	£500.00
5	4	D	£1,000.00
6	5	E	£100.00
Data

Excel 2010
	A	B	C	D
1	ID	Site1	Site2	Rate
2	1	A	E	0.25
3	2	A	C	0.5
4	3	A	D	0.25
5	4	B	E	0.8
6	5	B	D	0.2
SplitMatrix

Using the [Rate] in 'SplitMatrix' I need to return the following From 'Data'

Excel 2010
	G	H	I
4	ID	Site	Cost
5	3	C	£550.00
6	4	D	£1,065.00
7	5	E	£285.00
Query Result

Here is my current Query:

SELECT C.Site, SUM(C.Cost) As Total
FROM
(SELECT A.Site, A.Cost
FROM Data As A
LEFT JOIN SplitMatrix As B
On A.Site = B.Site1
WHERE B.Site1 IS NULL
UNION ALL
SELECT A.Site2 As Site, (B.Cost * A.Rate) As Cost
FROM SplitMatrix As A
LEFT JOIN Data As B
On A.Site1 = B.Site
WHERE B.Site <> NULL) As C
GROUP BY C.Site

I'm wondering whether this is the correct way to structure the Query or is there an easier way? (easier = fewer lines/more efficient)