Figure the price of supplies this way;

This will give you a round price when all added or multiply a single round x 20 for a box. If you have to buy brass, divided the price of the brass by the number of brass cases to get a single price for each casing, +
divided by the price of 100 primers to get the cost for one primer, +
divided by the price of 100 bullets to get the cost for each bullet or divide by 50 if packed this way, +
Divide 7000 grains (1 Pound) of pound with the number of grains of powder you will be loading to figure how many loads you can get out of a pound. Then divide the price of the powder by that number of loads from that one pound of powder.
Add up all the single round component prices (1 case, 1 primer, 1 load of powder and 1 bullet) for the single round price or multiply by 20 for a box price.