When sorting the register by category, subtotal too!

I just want to see some sort of subtotal. For example, I go to purchase for resale-inventory, which is under cost of goods, then I filter out the transactions the way I like (by time period, keyword, etc) I'd then like to see the subtotal of those transactions in the resulting view so I can quickly see or tell someone quickly when they ask, "oh, we made $16,432 in purchases for inventory for the month of February."