Moving Average Using Select Statement Or Cursor Based?

ID DATE(dd/mm/yy) TYPE QTYIN COST_IN_AMT COST_OUT_AMT(MOVING AVERAGE)
1 01/01/2007 PURCHASE 10 1000
2 01/01/2007 PURCHAES 5 1100
3 01/01/2007 SALES -5 *TobeCalculated
4 02/01/2007 Purchase 20 9000
5 02/01/2007 SALES -10 *TobeCalculated
5 02/01/2007 purchase 50 8000
6 03/01/2007 Sales -10 *TobeCalculate
7 01/01/2007 Purchase 20 12000

I have a table when user add new sales or puchase will be added to this table ITEM_TXNS. The above date is part of the table for a ProductID . (The field is removed here)
In order to calculate the balance amount using moving average, I must calculated the cost_out_amt first on the fly.
When user add new sales I also need to determine the cost/unit for a product id using moving average. The problem is I can not just use sum, because i need to determine cost_out_amt for each sales first which will be calculated on the fly.
The reason i dont store the cost_out_amt (instead calculate on the fly) because User could Edit the previous sales/purchase txn or Insert new sales for a previous date. Example THe record with ID 9. By Adding this txn with ID 9, would cause all the cost_out_amt will be incorrect (Using moving Average) if i store the cost_amout_out on entrying txn and need to be recalculated.
Instead I just want to calculate on the fly and able to determine the cost avr for a specific point of time.
Should I just use Cursor and loop all the record and calculate the cost or maybe I can just use on Select Statement?