When I made my bag a few years ago, I lucked out and got the fabric for $1.50 at walmart. Good luck repeating that find. I still ponied up and paid $60 for insulation from thru hiker to make it 20F. It was worth it...
Let's see, if I wanted to make it again with bought materials...
$50 for 7.5oz climashield
That's 2 yards.
$17 for 4 yards of ripstop
So $67 total, not including thread.
Using your yardage count, (3 yrds insulation, 6 yrds fabric), that would be $93 for a 20F quilt.
If you want it rated less than 20F, then the insulation cost goes down.
You can save money when you make it yourself, but that isn't always the case. The reason other companies can make it cheaper is economy of scale and cheap labor. They don't need a middleman to buy material from.