umm, 1000d cordura cost $14.00-15.00 a yard, and affected by the cost of oil. takes 3-4 yards to make a bag. thats $60 right there, 2 inch webbing is cheap actually, about .50 cents a foot takes 6 feet. then 1/2 webbing is about the same price.

mil spec thread for the pockets and seams is about $5-6 a roll if there is more than one color. my last bag had. 4 rolls so we are at about $90 total

then velcro and buckles so we will say $100

that's assuming you own pins, needles, a decent machine, scissors, ruler, other odds and ends.
And have already decided on a design

plus time spent. a hand done custom flap can take 3-7+ hours

everything is cheaper if you buy it in bulk, but thats assuming you have the cash on hand to buy +500 yards

metal hardwear, nicer fabrics of course cost more waxed cotton and duck is hard to come by for cheap.