Average for the last ten days repeated row by row

Hy everyone, I got a problem, I have this table

[BERLIN](
[QUA] [date],
[COST] [decimal](18, 2) ,
[REVEN] [decimal](18, 2),
[COSA] [nvarchar](50) 

I'm doing a lot of average, sum and count but now I'm locked with a problem. The table is so populated:

QUA         COST    REVEN         COSA

12/3/12    100,23    NULL          SHOES

12/3/12    1543,00  NULL          ADVERT

13/3/12     735,45   NULL          CAMERA

13/3/12     NULL     1035      SERVICE

......................................................

By this query:

;WITH CTE AS 
(
SELECT QUA, SUM(COST) AS COSTI, SUM(IIF(REVEN>0,REVEN,0)) AS RICAVI FROM BERLIN GROUP BY QUA)
SELECT QUA, COSTI-RICAVI as media FROM CTE

I got: date and average

............................... I 

I'd like to build a column with the average of the last ten days and I don't know if I have to try populating this other colum ( maybe using rowcount or the cursor) or if there is a way using the same query but I don't think so, surely I need you a loop here. Any suggestion? Thanks a lot