Inventory Valuation

I need to prepare a report on a periodical basis of inventory on hand Qty, value & age ( 0-30,31-60, 61-90 days etc.) In the database I will have few cost components of each purchase ( FOB, Duty, Freight etc.) by style #. Second table will be Sales(Date, Inv #, Cust, Style, Qty, Selling Px). I need to run the report usually on a month end but sometimes on any given date. Which functions/formulas I should use? I am bit confused. Some one please help with some sample codes/Functions/formulas. Thanks a bunch in advance!!