Need Help for a SQL Query

Question

I have a table sructure like below.

T_Sales

Bill_ID	Product_ID	Quantity	Price
1	A	2	$25.00
1	B	3	$78.00
1	C	1	$18.00
2	A	4	$98.00
2	D	5	$28.00
3	E	2	$105.00
4	D	3	$76.00

.

n-1           E

n              D

I need query for Total Price of each product_ID .

i.e,  Select Product_Id ,  Quantity*Price as Total Price

But, if same product is there in diff. bills, Total Price should be (quantity of A in Bill 1*price + quantity of A in bill 2 * price +....)/ (quantity of A in Bill 1 + quantity of A in Bill 2+...).

I hope, you understand my problem. Please let me know query for this requirement.

Thanks

Ranjith