J. Baker,
I basically have done the same thing for my cost of brass.
I keep track of the cost of the brass and divide it by the number of reloads per case that I have experienced.
I keep totals for each type of brass that I use for each caliber and divide the cost of that brass type by the total reloads for each type.

I keep my cost down by purchasing in large quantities.
I use Lapua brass because I get more reloads per lot (23 to 26 depending upon the caliber) than any other and it makes the real cost lower than some of the less expensive brass.
I buy bullets in the 500 per (I recently bought 2,000 Hornady ELD-Ms for my 6.5mm Creedmoor after Hornady offered the larger quantity lots.)
I always buy powder in 8 lb. lots, except for Alliant who only sells 5 lb. cans.
I buy primers in 5,000 lots as well.

There is a minor blip every time I buy another 100 brass or 500 bullets or 8lbs of powder OR 5,000 primers.
But after loading thousands of rounds in my favorite calibers, it hardly makes a difference in the totals day to day.

I keep the cost based upon purchase price for each item by every purchase and keep a running average for each item.
I have records of every bullet, powder, brass, and primer loaded for each caliber.
Yes, for 44,800+ reloads, it is an enormous spreadsheet but it lets me keep my cost records accurate.

Desert Dug,
I also have only a single press (Redding Big Boss II) with a RCBS 1500 Charge Master metering scale.
I don't think using a progressive press would change anything other than the initial cost of the more expensive press and the elimination of my expensive scale.
The powder, bullet, primer and brass used would be the same.
My costs of equipment, including 5 micrometer seating Forster dies for my most accurate rifle calibers, have grown to just over $2,200.
My cost of equipment was covered after about 3,500 rounds of the more expensive match calibers, which I started reloading with.

My totals for 44,852 reloads as of my last reloading session, show that my total savings, after subtracting the cost of equipment, is on the order of $25,196.