Calculate Price For Quantity

Hello all, I have an interesting question about calculating a price in my database for a quantity entered. I have a join table that I store ProductID, SizeID, Quantity and Price. The price for a particular product changes based most often on the quantity ordered. For example, if you order one unit of a widget the price is 10.00, however if you order one dozen units of the same widget the price drop to 9.75 and so on.

Here is a sample of my table....columns are in the order I specified above.

1, 1, 1, 10.00
1, 2, 1, 10.00
1, 3, 1, 10.00
1, 1, 6, 9.90
1, 2, 6, 9.90
1, 3, 6, 9.90
1, 1, 12, 9.75
1, 2, 12, 9.75
1, 3, 12, 9.75
1, 1, 24, 9.50
1, 2, 24, 9.50
1, 3, 24, 9.50
1, 3, 36, 9.30
1, 2, 36, 9.30
1, 1, 36, 9.30


So depending on if my widget is available in certain sizes, the second column, then each product has an price for the size id and quantity at which the price break occurs.

I use this stored procedure to return the price or price range based on the input parameters entered.

CREATE PROCEDURE dbo.sp_GetPrice
@ProductID INT,
@QuantityID INT = NULL,
@SizeID INT = NULL
AS
BEGIN
SET NOCOUNT ON
IF @QuantityID IS NOT NULL AND @SizeID IS NULL -- WE ARE L0OKING FOR A SPECIFIC PRICE BUT DO NOT HAVE A SIZE SPECIFIED
SELECT DISTINCT Price AS 'Price'
FROM join_ProductSizeQuantityPrice
WHERE ProductID = @ProductID
AND Quantity = @QuantityID

IF @QuantityID IS NOT NULL AND @SizeID IS NOT NULL -- WE WANT THE EXACT PRICE FOR THE SIZE AND QUANTITY SPECIFIED
SELECT Price AS 'Price'
FROM join_ProductSizeQuantityPrice
WHERE ProductID = @ProductID
AND SizeID = @SizeID
AND Quantity = @QuantityID
IF @SizeID IS NULL AND @QuantityID IS NULL
SELECT MIN(Price) AS 'Price Range' -- WE ARE LOOKING FOR A PRICE RANGE
FROM join_ProductSizeQuantityPrice
WHERE ProductID = @ProductID
UNION ALL
SELECT MAX(Price)
FROM join_ProductSizeQuantityPrice
WHERE ProductId = @ProductID

END
GO

So everything works great, however, when a user orders an quantity amount like 7, 13, 26 - which is not part of the table - I want to give them the discount available to them for the price break appropriate.

For example if a customer orders 16 widgets of size 2 the price break threshold they have crossed is one dozen, however they have not yet reached the next one which is two dozen. Therefore I want to offer the price associated with one dozen widgets of size id which is: $9.75. Once I have this a simple calculation of this price * 16 units would give me a total but my question is, how do I elegantly design this quantity / right price per unit calculation?

-Brian