DECLARE @t TABLE (FromDate DATETIME,ToDate DATETIME,priceusd Decimal(18,2),peak VARCHAR(32))
INSERT INTO @t
SELECT 
'2010-09-01' AS FromDate, '2010-09-15' AS ToDate, 1220.00 AS priceusd, 'High' AS peak UNION ALL 
SELECT 
'2010-09-16', '2010-12-16', 977.00, 'Low'

DECLARE @startDate DATETIME, @ToDate  DATETIME

SELECT @startDate = '9/1/2010',@todate = '9/21/2010'

SELECT *,DATEDIFF(D,fromdate, CASE WHEN todate <= @ToDate THEN todate ELSE @todate END ) AS NoofDays 
FROM @t

SELECT SUM(DATEDIFF(D,fromdate, CASE WHEN todate <= @ToDate THEN todate ELSE @todate END )*priceusd) AS NoofDays 
FROM @t