Based on the very vague details provided this is the in-line table valued function I would use:

CREATE FUNCTION dbo.ClientProductPrice (

@ClientNo INT,

@ItemCode INT,

@InvoiceDate DATETIME -- in SQL Server 2008 and later can be DATE

)

RETURNS TABLE

AS

RETURN(

WITH PriceRec AS (

SELECT

rn = ROW_NUMBER() OVER (ORDER BY PriceChangedDate DESC),

NewPrice

FROM

MyTable

WHERE

ClientNo = @ClientNo AND

ItemCode = @ItemCode AND

PriceChangedDate <= @InvoiceDate

)

SELECT NewPrice FROM PriceRec WHERE rn = 1

);

No promises that this will work as I didn't create a table to do any testing.